I learned my English grammar pre-CGEL and recently learned that there is regarded as a pronoun and as the subject of the verb in constructions like the following, where the verb agrees in number with the noun that follows it:
There remain traces of a canal system in the jungle.
There remains only the foundation of a grain silo on the long-deserted farmstead.
There are twelve things in a dozen.
There's twelve things in a dozen.
*There is twelve things in a dozen.
Why does the contracted form "There's" admit a singular verb with a plural noun whereas the uncontracted form "there is" does not? Why does coalescing "there" with the verb "is" allow the number to disagree?