Speedwell Cavern

Just over half a mile west of Castleton at the mouth of Winnats Pass, this claustrophobe's nightmare is reached by descending 106 steps for an eerie boat ride through flooded tunnels, emerging by a huge subterranean lake called the Bottomless Pit. Tours last 45 minutes; tickets must be pre-purchased online. New chambers are discovered here all the time by potholing expeditions.