Could a famous object on the floor of the ocean have gone undiscovered for centuries?

Question

In my setting, five meteors fell on the planet. Each meteor contained a very powerful magical gem.

Four of those fell on land, and the gems were retrieved rather quickly. The last one fell in an ocean, probably 5-6 kilometers (3.1-3.7 miles) deep.

Everybody on the continent saw the five meteors, and know that there must be a fifth gem. They know which ocean, and the very approximate location (maybe a zone of 50 square kilometers/19.3 square miles).

This need is not for immediate PC ability to find, but for world building and historical purposes. The meteor fall is a major event in my setting, and a lot will be based on this.

I want to make sure that it is realistic for the fifth gem to remain hidden for centuries, or at least long enough for everybody to get discouraged to search for it.

So basically, I need to know how fast the gem would have been found, and with which resources (spells, gear, etc.). If it's too fast, the story won't make sense, but if it's long enough, I can build a believable story around that.

Answer 1

Ultimately, whether or not it is possible or likely for the gemstone to be found is up to you, the DM, to decide.

But let's break it down a little.

Is It Possible?

Yes, the answers given thus far have provided a multitude of ways to find the gem - from object finding spells to special races that can survive underwater and even creative uses of animals and undead, there are many ways in a fantasy world to facilitate the finding of this object on the ocean floor.

Is It Likely?

Only if those resources are in the right place.

There's something important to note about all the suggestions made thus far about finding the gem - they all depend on access to either magical spells or fantastic creatures, and access to these resources is not guaranteed. Depending on where the gemstone landed, those who would have seen it may not have the resources to immediately find it, may not remember exactly where it touched down, may forget the event or write it down as legend rather than fact, or may just not be as interested as anyone else.

Over time, it would get less and less likely that the gem would be found - expeditions that fail to find the gem would pile up debt and go bankrupt, false leads, rumors and lies would create fog around the object's true location, powers interested in finding such an object would put their resources towards more certain pursuits of power, and over time the real knowledge and ability to find the gem more easily would be lost.

But It Is Up To You

But again, it is possible, and someone may very well have found that gem - they may have even kept their finding secret, fueling expeditions and legends that it could never be found.

When building a world, when building a setting, it is up to you to determine whether or not something happened. Knowing the plausibility of it occurring is important to know how amazing that event would be, but whether or not it actually did occur is entirely up to you.

Answer 2

Could it be possible? Yes, but it is up to you, the DM.

Assuming a magic world, there are many ways to search for this object, for example by narrowing the search area by using some divination spells. Other answers (see here and here) provided details for employing such spells.

In addition to divination magic, one may summon creatures for searching and retrieving the gem: for example, Summon Elemental is one of the best choices (details are below).

There are several strategies for finding the gem: the fact that it has not been found yet is completely up to you. For example, the gem is considered a message from the gods from underwater people (e.g., Merfolks or Sea Elves) and it is heavily guarded and/or hidden from divination magic. Or the meteor has landed in the ocean near a dead-magic-zone¹, where divination magic is ineffective and where summoned creatures cannot enter, or in a zone where magic has wild effects², where any magic could have bizzarre and/or unpredictable results.

Another possibility is that the meteor has landed in a particular zone of the ocean, but during the centuries strong underwater currents eroded it and made the gem travels for mile: people have been searched for this gem in the wrong place.

Summing up, if your story needs that the gem is a legendary item that has not still been found, you can come up with several reasonable settings, which can be also heavily involved in the quests/campaign for the search, if you will.

Down below a short list of strategies for searching and for retrieving the gem is provided, beside the divination approach listed by other users.

Be (or hire) a druid.

Another way to search for the item is to be (at least) a 4th level druid that had seen a Dolphin (or another beast with swimming speed): in this way the druid can search the area and find the location of the gem. In case the subclass is the Circle of the Moon, there is no CR restriction.

When the druid can wildshape into a Giant Octopus, they could be able also to retrieve the gem, using the tentacles.

The drawback of this approach is the depth at which the gem is. For example, if the gem is at more than 500 meters deep, in real world dolphin cannot reach it. This approach could be of some use in case the gem is "close" to the surface, or to spot unusual location in the search area from far away. About Giant Octopus, this is up to the DM.

The DMG provides some rules about underwater pressure:

Swimming through deep water is similar to traveling at high altitudes, because of the water's pressure and cold temperature. For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.

For depths as the ones depicted in the answer, the DM has to decide if the above rules still hold or if more dangerous arise, or they can decide to adopt and adapt the optional rules from Ghost Of Saltmarsh:

OPTIONAL RULE: PRESSURE AND OBJECTS

With this optional rule, characters who dive deep in the ocean require specialized equipment that can withstand the ocean's pressure. Nonmagical objects not made to withstand the water pressure are destroyed at various depths, as determined by the material used to create them. This destructive depth is presented for various materials on the Objects and Water Pressure table. Objects made of other materials break at the DM's discretion.

Material	Destructive Depth
Glass. crystal, ice	100 ft.
Wood, bone	500 ft.
Stone	1,000 ft.
Iron, steel	1500 ft.
Mithral	2,000 ft.
Adamantine	2,500 ft.

Another possibility is to borrow the rules from Storm King's Thunder³, chapter 10, page 202, when the Maelstrom is described:

Maelstrom lies on the floor of the Trackless Sea, nearly 3,000 feet beneath the surface.

[...]

Creatures and vehicles at Maelstrom's depth take 7 (2d6) bludgeoning damage per minute from water pressure unless they are adapted or built to withstand this environment. [...]

A spell that allows one to breathe underwater provides no protection against the crushing effect of water pres­sure unless the spell's description says otherwise.

In case of a Circle of the Moon Druid⁴, at 10th level one can wildshape in a Water Elemental: this make the search more easy.

One can interact with marine beasts, using Speak with Animals.

The Speak with Animals spell allows to, well, speak with animals: intelligent creatures such as dolphin can be employed for such task, making them explore the area for you and report where the gem is. Moreover, if the gem is not stuck anywhere, the caster can instruct the creature(s) to retrieve it for them:

You might be able to persuade a beast to perform a small favor for you, at the GM's discretion.

All the limitations about pressure and depth are still to be considered.

Summon creatures for doing the job for you.

A couple of spells can employed: Summon Beast, for summoning a marine beast, and Summon Elemental, with the natural choice of water. The caster can instruct the creatures for searching and eventually retrieving the gem (in case of beasts, see the remarks in the previous section).

A remark about Water Breathing.

Water breathing allows to breath underwater, but at 5 kms (5000 meters) underwater, the pressure is around 501 atmospheres, deadly for normal humans.

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¹ Have a look at the box "The Weave of magic" in the PHB:

[...] the most powerful archmage can’t light a candle with magic in an area where the Weave has been torn.

See also the box Weave-Affecting Magic in The SCAG:

Dead Magic. In rare areas of dead magic, the Weave is absent. Not only do spells and magic items cease to function, but even the supernatural abilities of creatures that are innately tied to the Weave might fail as the knot of the Weave they carry with them unravels.

² See again the box Weave-Affecting Magic in The SCAG:

Wild Magic. In an area of wild magic , the Weave becomes "tangled," spontaneously forming its own constructs and resulting magic. It also tends to twist the constructs of the Weave created by spellcasting, causing unexpected results.

³ Credits to Peter Cordes that pointed this out in the comments.

⁴ Credits to Tollef that pointed this out in the comments.

Answer 3

Yes, this gem would have been found

If there are civilizations of water creatures living in the deep ocean (Sahuagin, Sea Elves, Tritons and so on), they could have found the gem just as easily as your land-based culutures did on land. Even if not, there are several strategies that could have been used by observers who have access to magic to find this gem in a rather short time.

Given that the spells involved here would allow a single, mid-level character to find the gem within a week, and that you describe the gem as a very powerful magic item that would be highly desirable for spellcasters or powerful rulers, it seems very unlikely the gem would not have been salvaged just based on being hard to find.

You will need to establish other hurdles for this to be believable. For example, it might be in an area that is extremely dangerous due to sea monsters like a Kraken. You as a DM also could come up with excuses or justification as to why it has not been found if that is important for your plot. Maybe it fell into a dead magic zone, so some divination spells cannot directly touch it; maybe the gods don't know about it for some reason, or have a deal or reason to not disclose anything about it. Maybe that it cannot be found by anyone who is actively searching for it is one of its magical properties.

Example Search Strategy

One way to find it is a binary search strategy with commune (thanks to @PeterCordes for pointing out this is better than divination or augury, that work along similar lines):

You contact your deity or a divine proxy and ask up to three questions that can be answered with a yes or no.

You move to the middle of the ocean area and ask: "If I will search the ocean floor East of here tomorrow, do I have a chance to find the fifth meteorite?"

A fifty square kilometer area is about 7 kilometers long per side. After you receive your answer, you just halved your search area, to 3.5 x 7 km. You then ask again: "If I will search the ocean floor South of here tomorrow, do I have a chance to find the fifth meteorite?" After that day, your area will be down to 3.5 by 3.5 km.

To avoid the risk of wrong divinations, you can cast the spell only once per long rest. It would be up to your DM if the gods humor multiple, different people casting it on the same day. Plainly rules-as-written, the limit probably applies per caster, but lets assume you do this with a single caster. As you have to ask all questions within 1 minute, you might not get to the middle of the now quatrered area to ask the third one. So you cast the spell again the next day.

You continue doing this halving and quartering of the area strategy. After half a dozen days, you are down to about 300 feet by 300 feet.

@ObliviousSage asked on Wordlbuilding SE about the possible deviation scatter once the meteorite hit the water. It looks like this would roughly double the search area to 95 square kilometers. As this strategy halves the area every day, that would mean you search one day longer, at most 7 days.

There are no rules for deep water pressure that make it hard to survive in depths in 5e¹, and D&D is not a physics simulation, but our DM might invent some rules that say you need to deal with such pressure, and it might be hard to survive, even if you can breathe water. In that case conjure elemental a water elemental to go down there and retrieve the gem. If you are concerned about how long it takes to travel down that far, you can use planar binding to extend the duration.

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¹ Thanks to @KorvinStarmast's pointers: there are some rules on deep water environments. The first, from Unsual Environments (DMG, p. 116), merely increases exhaustion from extended exercise if you lack a swimming speed (so using a ring of swimming would entirely remove it as an obstacle):

For a creature without a swimming speed, each hour spent swimming at a depth greater than 100 feet counts as 2 hours for the purpose of determining exhaustion. Swimming for an hour at a depth greater than 200 feet counts as 4 hours.

Ghosts of Saltmarch has Optional Rules that destroy various objects due to pressure at a given depth, based on their material, from glass at 100 feet depth to adamantine at 2,500 feet.

Answer 4

Could a famous object on the floor of the ocean have gone undiscovered for centuries?

Sure, but I suspect this is not your real question.

If it's too fast, the story won't make sense, but if it's long enough, I can build a believable story around that.

I suspect this is your real question. You want the gem to be findable but the story believable.

The simple way to do that is let the players be special. That can be everything from powerful to lucky, smart to connected, desperate to determined. The kind of special that only shows up every few centuries.

Players have free will. So there really is no predicting how they solve any puzzle. You can give a chest an expensive trapped lock and hide the key at the center of a Minotaur maze. One that took some effort to populate and was supposed to last 4 weeks of gaming sessions. Then watch as your players break open the chest by throwing it off a cliff.

Rather than try to predict how long it will take, think of many ways to approach finding the gem. Create things that make each of those ways difficult. Not impossible. Just difficult.

Could be the gem didn't just land in the ocean. It landed next to a Kraken that thought the gem would make a nice addition to it's nest.

Could be the local sailors have known to sail clear of the Kraken for generations. They want no part of the party's fight with it.

Could be the gems power is invisibility. And well, it likes being invisible.

Could be the meteor hit the side of an undersea cliff so it's not only under water, it's under rubble.

Could be the meteor was made of unobtainium and some dwarfs already looted the site for the rare metal. They didn't know what the gem was and so used it to decorate their door knob.

Thing is, you could have all of these ready to go. The idea isn't to overwhelm the party with all of them. Just keep the tease going long enough to keep things entertaining. Even if they find a way to shove your Kraken off a cliff.

Answer 5

It could be next to impossible to find

I'm assuming what you really want to know is "How is it possible for it to not be found?" instead of "Could it be possible?" Ways to reasonably explain the setting that you want to create. So here is a buffet of overlapping factors from which to pick, which can easily account for the gem not being found.

1. Historical knowledge is vague, and vagueness is your friend. This applies to PCs and NPCs. "We saw five meteors so there must be five gems" is awfully clean-cut, and imho unrealistic. For crying out loud, just the other week there was a U.S. Congressional hearing regarding UFOs. In the 21st century. The Night of Sky Fire was a long time ago, and accounts vary from the very beginning. Was it really at night? Were there really five? Some saw one or two meteors, but every old gossip at every tavern swore they saw a dozen.

2. There really were a dozen, or hundreds of meteors that fell that night. It happens that only four gems were found. After expeditions into increasingly dangerous, inhospitable (and thus expensive to search) locations with no others being found, the interested parties are no longer willing to fork out the gold to try further. (Also, even one meteor that's large enough to see well from any given place on an entire continent would probably cause an extinction event. Smaller and more numerous avoids this, and makes the event more visible.)

3. Many more gems came from said dozens of meteors. The Four became legendary because they were exceptional in size, quality, and magical power, but there's not much reason to presume a fifth of such quality exists after not finding it for a century or so. This scenario makes Locate Object, which may find "the nearest object of a particular kind," give too many false positives on the more common, tiny gems to be useful in finding the fifth big gem.

4. Impediments to magical discovery. The fifth meteor had a very high lead content, such that Locate Object and Detect magic spells are fruitless. It settled into a long-forgotten anti-magic field. It settled in a place both dark and deep, so Clairvoyance helps neither surface nor underwater casters.

5. An underwater race did indeed find the gem, but the secret order they formed to protect it died out. Perhaps it became too small and secretive, and happened to be killed off during a plague or coup against the reigning king. Or perhaps it still lives and is just very good at its job? Either way, the gem resides in the ruins of a forgotten underwater temple with the aforementioned magical defenses in place, far from where the impact site would have been.

I need to know how fast the gem would have been found

That is entirely up to you creating the narrative. It could lay dormant for untold centuries before some merkids go into the scary chasm on a dare and stumble across a weird-looking rock. If, however, someone looking for it finds it centuries later when no one else could, you would naturally want to justify that with a precipitating event: perhaps a map from the secret order surfaced, or an underwater earthquake shifted its position.

Answer 6

Water breathing and locate object could do it.

With level 2-3 spells, any interested parties could easily search 1000 feet regions for meteors which they were familiar with. Consensus was you can cast underwater, so long as you can breathe. It would take a while, but you could certainly do it. You can replace water breathing with a magical item or being a triton, a sea elf or such. You'd expect them to find it within a couple months since they can just swim down, cast the spell, and swim up to regenerate spells.

Since this just requires level 3-5 characters (only level 3 with water breathing races, as locate object is a level 2 spell), I wouldn't expect that in a few centuries no one would be able to do it- level 3 isn't a huge barrier. You'd have to have some sort of force blocking such attempts, like a hostile sea kingdom around the meteor.

Notably, 50 square kilometers is a fairly small area, so the 1000 feet scanning range of locate object could quickly find it.

The maths of how to find it.

A circle of 1000 feet has an area of 3141593 million square feet. 50 square kilometers is about 538195520 feet. 538195520/3141593=171 spell castings. Of course, circles are not a perfectly efficient way to search because the circles have gaps, and the deep ocean may be difficult to search and mistakes may be made, and you need to make the circles a bit closer so they can search under the sea bed, but a level 5 spell caster has 3 level 2 spells a day, so they can on their own easily complete this in under a year.

A basic search pattern would be to cast water breathing on you and your party, swim down with your party to the ocean floor within your 24 hours of spell use, mark out five centers of circles as such with heavy stone or copper anchors or by having a party member using a cantrip like mold earth, or by moving local rocks.

And then cast your 3 spells, with one level 3 spell reserved for emergencies. Mark each anchor as soon as you search it, and repeat. Even if the meteor is buried beneath the ground you should be able to find it. It might take a while, perhaps months or even a year, but it's not a very difficult task. Not centuries difficult.

Answer 7

Some classic options:

Divination:

Your magic and an offering put you in contact with a god or a god’s servants. You ask a single question concerning a specific goal, event, or activity to occur within 7 days. The GM offers a truthful reply. The reply might be a short phrase, a cryptic rhyme, or an omen.

Ask your DM where you can find the meteor, or alternatively, state you're going on a 7-day journey to find the meteor in the ocean, and ask them where to go.

Locate Object:

Describe or name an object that is familiar to you. You sense the direction to the object’s location, as long as that object is within 1,000 feet of you. If the object is in motion, you know the direction of its movement.

Do note here the 1,000ft description, which Nepene's answer addresses.

Legend Lore:

Name or describe a person, place, or object. The spell brings to your mind a brief summary of the significant lore about the thing you named. The lore might consist of current tales, forgotten stories, or even secret lore that has never been widely known. If the thing you named isn’t of legendary importance, you gain no information. The more information you already have about the thing, the more precise and detailed the information you receive is.

The definition of "Lore" is also relevant:

a body of traditions and knowledge on a subject or held by a particular group, typically passed from person to person by word of mouth.

Assuming (1) the meteor is legendary or has "significant lore", and (2) your DM would treat the location of the meteor as "lore" about the meteor, Legend Lore may also help reveal its location.

Answer 8

As various people have demonstrated, the use of divination magic will quickly find the gem.

Thus, if you want the gem to not be found you'll have to hide it from divination magic in some fashion. The only reasonable explanation I see is that it was found by something living in the deeps that is now hiding it.

Details? Others have addressed why it would be found. Thus you're asking for designing a monster or NPC to be hiding it!

Answer 9

There are various obstacles, some of which do have solutions, others of which could combine to be enough of an obstacle, depending on other world-building / magic-mechanics choices. Like how much the gods know about precise details of where things are (Commune / Divination), and how common high-level magic-using characters are, especially ones who might team up with a druid to go after the gem.

• Travel time from/to the surface: longer than the 10 minute duration of Locate Object unless you teleport. Even a water elemental dashing every round takes 9.1 mins to go down 5 km.

  Teleportation can help if you have it, or Transport Via Plants if there's a large enough "plant" in the inky blackness of the deeps. Perhaps set up an underwater garden with an artificial daylight source, so you can go back and forth.

• Locate Object can't be given a category like "magic gem", and a specific object like "the meteorite containing the gem" can't be specified unless you've seen that specific object up close. (Including by magic like arcane eye or scrying). (Previous discussions include Defining "a particular kind" in the Locate Object spell and a Reddit thread.)

  You could use it to find meteorites in general, maybe with a size threshold, which might lead to having to deal with some false positive hits (cover with lead sheet or bring up and sell). Or maybe locate "fist-sized gems", or "red gems" if all 4 of the known gems are the same colour. This depends on how magic works in your campaign, like how useful / powerful you want Locate Object and divination magic that doesn't communicate with higher beings to be.

• Casting spells at the crushing pressure of depth: humanoid forms will have their chests crushed since the air in their lungs only pushes back at 1 atmosphere. You can't cast spells while wild shaped or polymorphed. And Shapechange (9th) is self-only and concentration. Locate Object is also concentration.

  True Polymorph (9th, bard/wizard/warlock) doesn't rule out casting spells, but the target doesn't keep their own spellcasting abilities as that's part of their "game statistics", so they'd need to cast from an item they could still use without their class. True poly into a water elemental that's immune to exhaustion and can dash for 180 feet is pretty good. Even for just searching visually if you give them a bright light source so better than their 60 ft darkvision. And give them a way to mark areas they've already checked.

• Gods might or might not know the precise location of the meteorite (or gem if something else recovered it and has it hidden). If so, they can answer Commune (5th, rit.) questions like "is the artifact gem north of my current position"? Or "is the artifact gem north of marker X I left on the ocean floor near my ship?" With a water elemental servant to move it around as you narrow things down, you don't need a PC to actually go down there. Sending (3rd) could communicate with such a servant, so they could move the marker before another cleric you're allied with casts another Commune.

Casting spells at depth

A 20th-level druid can cast Locate Object while wild-shaped. Lower level druids would need an item they could activate, if they can't survive being in humanoid form at depth. Locate Object has material components so level 18 druid Beast Spells doesn't help.

Or be Shapechanged (9th) into something that could survive the depth and still cast, but Shapechange is concentration and self-only. Maybe a 17th-level moon druid could cast Locate Object (dropping concentration on Shapechange) while simultaneously using wild shape as a bonus action into something that could survive the depth (e.g. water elemental or giant octopus), being humanoid for such a short time that their lungs don't crumple.

Searching the lightless depths without magic would be very slow for any significant area. (You can bring a magical light-source, but you can't look around and see anything outside its area of Bright Light that you're in.)

If Commune (5th, rit.) works well, and you can reliably find a place on the ocean floor (e.g. by dropping a hugely long anchor cable and moving a marker around?) you can progressively narrow the search area until it's small enough to comb manually. In that case a 9th-level Cleric would be able to narrow the area enough for a 4th-level Druid to comb it even without Locate Object. (Preferably higher level, preferably moon druid 10th for water elemental form.)

So for the item not to be found, you probably need the major deities (and their proxies that answer commune questions) to not know the exact pinpoint location of the gem, or the meteorite containing the gem. This has world-building implications, and gameplay in terms of what kinds of questions Commune can answer about other factual information, unless there's a special reason why the gods don't know this but do know other random specific facts about locations of things.

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The depth makes it non-trivial even for druids.

Even just travel time from the surface and back is very significant, longer than the 10 min duration of Locate Object (2nd, VSM). 5km is about 16404 feet. At a swim speed of 40 ft, that's 41 minutes one way. Or at a swim speed of 60 ft, 27.3 minutes. As a Water Elemental (swim 90) dashing every round (immune to exhaustion), that's 9.1 minutes. (Even a crab holding onto a heavy rock won't be that fast.)

A 4th level druid can remain in wild shape for 2 hours, and you can short rest while wild shaped. But if you try to long rest, you'll need a wakeup call every 1.75 hours to avoid reverting to your humanoid form and being crushed. (A higher level druid would have much less of a problem, e.g. a 10th level druid could get almost 5 hours of sleep, long enough for a sustainable sleep cycle of sleeping about 4 hours, renewing wild shape, and going back to sleep, waking up with 2 uses of wild shape left since each half of your 8 hour long rest should also count as a short rest. Or if you're a moon druid wildshaped into a water elemental, you're immune to exhaustion. If the DM decides that means you don't need sleep at all, you can just short-rest.)

The 1000 ft range of Locate Object is only about 1/16th of the depth, so you're only in range of the bottom for a small fraction of the travel time.

And druids can't cast spells while wild-shaped until 18th level. And not with material components until level 20. (And no, a feat for subtle spell doesn't solve this problem.) Shifting back to humanoid form to cast, their rib cage would be crushed by depth. There are beasts that should be fine with the pressure of very deep water, like octopus (CR0) or giant octopus (CR1).

Arguably with Water Breathing, your lungs can be full of water so your rib cage wouldn't be crushed by the pressure, but other cavities in your body (like sinuses) could implode. Bones themselves might be ok under external pressure that's uniform in all directions (including through the watery fluid of the body.)

A 17th-level druid with Shapechange could be in an animal form for an hour, but unlike True Polymorph it doesn't become indefinite until dispelled if you concentrate full duration. And it's concentration. But you can cast non-concentration spells while it's active, in case that helps, if the form you shapechange into is capable of performing the verbal and somatic components. (And you can pop back into wild shape after. Going from wild shape to Shapechange is a problem, though, if you aren't a lvl20 druid to cast a VSM spell while wildshaped, but then you wouldn't need shapechange.)

True Polymorph doesn't rule out spellcasting, but you don't keep your game statistics so you could only cast from items you brought with you. Then the wording of "* can't speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions*" becomes relevant. (Or if the form you shift into has the spellcasting or innate spellcasting features, but there aren't underwater creatures with Locate Object.) The Morkoth has a swim speed of 50, darkvision 120 ft, and a passive perception of 20, and can breathe underwater. 3/day Sending can let it communicate with someone on the surface casting Commune.

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The biggest obstacle to higher-level characters not finding and retrieving it is mid-level divination magic, specifically Divination (4th, rit.) and Commune (5th, rit.). If a being that answers Commune questions knows where it is, someone can narrow things down pretty quickly.

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P.S. I started writing this answer a couple weeks ago to collect up stuff I and others had discussed in comments. I kind of got bogged down and didn't have a conclusion, but I guess I'll post what I have after some tidying.

Answer 10

I've read or scanned the current answers, most of which focus on why it would be found. I'd like to answer with a couple of reasons as to why it might not be found. The first on the science of why it might not be exactly where people expect it to be, and the second on the speculation as to why it may not be found by some of the methods people have posited as to why it WOULD be found.

The Science As the meteoroid enters the atmosphere, it starts to heat and gets really hot. Inside are gases that become super-heated and, eventually, create such a pressure as to blow the incoming meteoroid apart into smaller chunks. The people have known of this event for centuries, but, possibly much like the dark ages, did not have the maths to calculate trajectories. What they assumed happened, based on where the other four landed, is that the fifth landed in the sea within a general area as you have mentioned. What actually happened was that:

1. The first gas explosion split the meteoroid apart in a roughly 60/40 split (the actual amount is somewhat irrelevant).
2. Based on an irregular break, the smaller portion started to slow and fall behind the arc of the meteoroid as it progressed through the atmosphere. This can readily be due to density and or aerodynamics of the piece.
3. At some stage, the smaller portion broke apart again, quite explosively, projecting one half further forward, while decelerating the other half. This is the lost fifth.
4. The other, larger 3/5 of the original break has also subsequently broken apart two - once in 1/5 and 2/5 pieces and then finally that 2/5 into two separate pieces. You now have five individual pieces.
5. Based on the pattern of how the four portions that landed on land are dispersed, people who saw the entry and knew that there were five pieces, searched along the same entry path, not knowing that the explosive separation of the smaller initial break had pushed the unfound piece further off course - behind or to the side.

Fun fact: Your space rock is actually a meteorite once it lands, not a meteor or meteoroid. NASA

The Speculation I have three unproven ideas as to why the gem was not found so quickly:

Firstly, the last gem may have landed in a deep trench (e.g. Marianas-like, close to land). This could easily be a continental trench very close to the main continent where the other four gems landed.

Secondly, the gem could have been embedded in an underwater hill or trench cliff side, covered in detritus due to an underwater landslide caused by the impact and only recently been eroded clear.

Finally, the reason that it may not have been divined, as other answers suggest that it would have, is that it could be a new element/magic and/or that it actually isn't in the area they have been looking for it in.

You haven't really said what technologies or magics your world-building story will have, so it may easily not be divinable anyway.

[EDIT]: My tired eyes missed the (now obvious) tag for DND 5e. I then deleted this answer before undeleting it because, while much of my answer will not apply to DnD, some of it still very much applies to the plausibility to the answer.