Midnight's Lair is a novel by Richard Laymon first published in 1992. It was republished with Endless Night in Volume 9 of the Richard Laymon Collection.
Summary[]
Mordock's Cave is one of the 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 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...