Kaumana Lava Tubes

Natural lava tubes crisscross subterranean Hawai'i, but few are both easily accessible and relatively devoid of tourists.  

The Kaūmana caves offer an alternative to the Disneyfied lava tube experiences found elsewhere on the island: the caves, located in a tiny park without any formal supervision, are open 24 hours a day, year-round, and are free to enter—but since they are on the far side of the Big Island, away from most of the tourist scene, they aren't overrun with foot traffic.

Lava tubes are created when the topmost layer of a lava flow solidifies, but an undercurrent of lava continues to flow beneath it.  When the lava drains away entirely, the hollow core remains—and can be accessed if the roof caves in, as has occurred at Kaūmana.

Sunlight only trickles a few dozen yards into the tubes, but the formation extends much deeper, and is navigable by anyone with reasonable hiking skills and a flashlight (no artificial light has been installed, so bring a flashlight or hope that someone has kindly left one at the mouth of the cave).  The rocks may have been worn down a bit over the years by your spelunker predecessors, but they're of volcanic origin and consequently are sharp, uneven, and in some cases precariously-balanced.  There are no significant pools or streams inside the tube, but because the air is very cool and humid, all the rock surfaces have a sheen of moisture on them, and can be slippery.