Black Chasm Cavern was designated a National Natural Landmark in 1986 by the National Park Service, due in part to its incredible display of rare helictite formations. It has been open for public tours since 1998.
Cave speleothems are generally formed by dripping or splashing water, but helictites are formed through hydrostatic pressure. Basically, water is pushed through porous walls leaving crystallized minerals on the other side. As the mineral deposits build up they tend to stack horizontally resulting in the incredible spun glass effect that can be seen in the Landmark Room on the walk tour.
Part of the tour takes visitors across a bridge giving small glimpses of the naturally blue lake below.
Guides talk about the early history of the cave and its unique geology. This is a fairly easy tour although there are quite a number of stairs involved and its definitely worth seeing how beautiful the Sierra Nevada are on the underside!