Way Down Atlantis

From the introduction:

When the navy boys finally found the shattered hull of the missing American submarine, they found something else, too. The UFO people and the other fruit cakes think the navy found Atlantis, and the academic types have more theories than you can shake a stick at. I didn’t care what it really was, it was just another deep job for me and Leviathan Deep Operations. At least it started out that way...