I suggest that Jesus' command to "this mountain," "Go throw yourself into the sea," holds a clue to which mountain Jesus meant. The evangelist Mark also used the "cast into the sea" imagery in another, earlier episode.
In chapter 5:1-20 appears the story of Jesus healing the man possessed by many unclean spirits, called Legion. There, when Jesus exorcised the demons they invaded a nearby herd of pigs (2000!) that promptly rushed down a steep bank and into the sea.
Did this strange, hyperbolic episode actually happen, or did the evangelist construct an allegorical story full of symbolism to mask an uncomfortable fact, that at some point in his ministry Jesus had preached that it was time for the occupying Romans to be driven back into the Sea from whence they came? In other words, was he preaching sedition? If so, it's no wonder the people of the region begged Jesus to depart. His presence and message would likely have brought about their destruction by the Romans.
Dr. Harvey Cox provides a similar interpretation in his book "The Future of Faith," pages 69-70. He refers to an "anti-imperial quality" in early Christianity, and reflects on the collective name of the demons being "legion, which everyone who originally heard the story would have recognized as a squadron of occupying Roman troops. Both the boy [man] and their homeland were occupied by foreign bodies from which they needed to be liberated."
https://en.wikipedia.org/wiki/Harvey_Cox
In Mark 11:23, when Jesus commands "this mountain" to cast itself into the sea, I believe he is referring to Mount Zion/the Temple Mount. I see this teaching as a counterpart to the episode of Mark 5:1-20. In chapter 5, Jesus preached about overthrowing the Roman occupiers. In Mark 11:23, Jesus preached about overthrowing the corrupt Jewish collaborators entrenched in God's holy temple. Both groups were oppressors of the Jewish people, and they were destined for the sea of their own destruction. They were not to be inheritors of the coming kingdom of God.
Mark 11:23 is part of a longer cycle that starts at the beginning of chapter 11 and concludes with the parable of the wicked tenants and the vineyard, Mark 12:1-12. All parts of the cycle underscore Jesus' message that the current Jewish power structure - chief priests, elders, scribes - was to end. The time was fulfilled, the kingdom of God was at hand.
At the beginning of the cycle Jesus entered Jerusalem on a donkey/colt to joyous acclaim. He was finally revealing, symbolically, that he was the long-anticipated Messiah. That day, he inspected the Temple and then left. Next morning, he cursed the fig tree in full leaf but fruitless, a symbol of Israel's leaders, who gave the appearance of righteousness, but were not righteous. He next cleansed the corrupted Temple, the "den of robbers." The following morning, his disciples marveled at the shriveled fig tree.
This led to Jesus giving instructions to command "this mountain," the occupation force on the Temple Mount - the Jewish leaders in the Temple and the Roman presence in Antonia Fortress - to cast itself into the sea.
Through his words and actions, often couched in symbolism, Jesus proclaimed that he was the awaited Messiah, come to deliver Israel from its oppressors. His mission was to usher in the kingdom of God. That meant the status quo had to go. No wonder the authorities, Jewish and Roman, wanted to execute him.