6Moaning Cavern Stairs, California, USA

The bones of approximately 100 prehistoric humans were once found at the bottom of these stairs, in this largest single-chamber public cave in California.

To reach this cave, which is big enough to fit the Statue of Liberty in, climbers must descend 235 stairs, 144 of which are on a spiral staircase. This damp cave is known for its eeriness, sounds of moaning and wailing are often heard as visitors make their way down.