This story strikes me rather like the sort of thing they liked to write 70 or more years ago - and I'm not talking about anything that would be considered a classic. The word-weaving may be better than old pulp stories, and its veneer is more modern. Nevertheless, the basic storyline is old-as-the-hills pulp. You know, the days of lost continents, an inaccessible land in the middle of nowhere, ancient cities on the sea bottom, vast caverns, or anywhere else you might misplace an entire world.
In this case, it's a civilization conveniently tucked away below the sea floor of the Atlantic Ocean. A deep sea drilling operation stumbles across something mysterious at the bottom and an investigation ensues. Just about the time the investigators are discovering the entrance to a realm beneath the sea floor, circumstances force them to go into what turns out to be this hidden realm.
As the story evolves, it turns out the realm's hidden people want to stay hidden and unknown by the rest of the world. So the investigation team is not allowed to return home. It's not just that the hidden people want a quiet, cloistered life - they have interstellar spaceships that they manage to send out from the interior of the Earth and other super-duper stuff. These people had once lived on the surface before an impending catastrophe eons ago led them to move downstairs to safer quarters. After the disaster, life on Earth's surface started over from scratch again, and oddly enough produced humans who look just like the hidden people below, how quaint.
The investigation team is kept in rather comfortable detainment, but they want to return to families and everything they know back home. So they decide to escape...
If you like old "lost race" stories, or maybe even if you just like something no more demanding than Harry Potter, perhaps you would like this. Otherwise, I recommend you skip it.