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