Abduction
Oceanographers in Atlantic Ocean are sucked in to become trapped inside an ancient undersea volcano by a race of technologically advanced humans living there. These folks preceded the modern day humans by many millions of years.  They sought refuge in the middle of the earth [Mohorovičić discontinuity] when they discovered that the earth was about to be struck by a multitude of meteors – effectively wiping out life above ground.  The Moho folks shared that they had watched the next generation of humans [us] evolve  and had abducted the oceanographers to determines if the surface folks were looking for them.  
Interesting interactions and plot development. Unexpected climax. 
A good read
Fiction, Oceanographers, Ocean bottom, Large type books, Open Library Staff Picks
Robin Cook
Dr. Robin Cook (born in New York City, New York) is an American physician and novelist who writes about medicine and topics affecting public health.

He is best known for combining medical writing with the thriller genre. Several of his books have been bestsellers on the "New York Times" Bestseller List. Several of his books have also been featured in Reader's Digest.