Midnight's Lair

This book was later published as by Richard Laymon.

Mordock's Cave is one of the wonders of the world: a place where thousands of sightseers every year take an awe-inspiring trip on a lake deep beneath the earth's surface and marvel at Nature's handiwork. But the darkness is also the home of things Nature never intended – things violent, bestial and obscenely evil. And when a sudden power failure traps a group of tourists underground, the unnatural creatures who dwell there emerge from the depths...