A very long time ago I read a paperback at school, it was one of if not the first SciFi books I ever read. I would like to try and find a copy but being so long ago I have no recollection of the author or title.
As I recall the plot involved the following.
A couple of divers discover a sarcophagus hidden in an underwater grotto. It contains a woman in suspended animation. She is from a long lost city which may or may not have been Atlantis. (I can't remember.) They bring her sarcophagus on to their boat and revive her.
Later in the book they discover her city still intact deep underground. They also discover still living an enemy race of the woman's civilisation. The enemy may or may not have been humans, more likely not.
I think it ended with the enemy being defeated but at the cost of the buried city either also being presumed destroyed or lost by descending even deeper in to the Earth.