Alabaster Caverns State Park in northwestern Oklahoma centers on the world’s largest natural gypsum cave open to the public, notable for alabaster gypsum in white, pink, and rare black forms as well as selenite crystals. Formed from Permian mineral deposits left by an inland sea and later carved by water, the main cavern extends about 0.75 miles. Known in the 19th century and first documented in 1898, it developed into a public attraction after 1939 and became a state park in 1956. The caverns have also served as a fallout shelter and remain important bat habitat, while guided tours continue despite a major 2018 entrance collapse.