[Bouncer]

Also, check Jonathan Gray's site for stories about ancient undergroud tunnels, and whole cities:
http://www.archaeologyanswers.com/

There is an older collection of tales called the Underground Empire:
http://www.v-j-enterprises.com/undergnd.html
This one tells of the secret expeditions by the Navy into the undersea passages at the coast of California.

If someone drove a pirhana-class sub into those caverns with a nuke . . .