Midnight's Lair — Richard Laymon

Mordock's Cave is one of the spectacular wonders of the world, a place where thousands of sightseers every year take an awe-inspiring boat trip on a lake deep beneath the earth's surface to marvel at Nature's handiwork. But the darkness is also the home of things Nature never intended - things violent, bestial, and obscenely evil.
When a sudden power failure incapacitates the elevator to the surface, a group of tourists is trapped in the underground depths. Cold and scared, without lights or food, their only hope is to find an escape route through the sealed-off end of the cavern. But their explorations uncover a nest of horrors that has lain hidden for generations, and their idyllic underground journey becomes a nightmare trip through hell, as they find themselves battling for survival against the creatures of the abyss....