Illinois Caverns State Natural Area in southwestern Illinois preserves the state's second-largest cave, a karst system with more than 9.6 kilometers of passages still shaped by water dissolving limestone and dolomite. Known by several historic names, it entered public use in 1901 and became Illinois's only commercial cave attraction, drawing visitors during the 1904 World's Fair before closing in 1907. Sold to the state in 1985, the site is also ecologically important, supporting bats and other cave life, including the federally endangered Illinois Cave Amphipod. Access is now regulated, with permits and equipment required after a 2010 closure tied to white-nose syndrome and a 2021 reopening.