Twin Caves in Spring Mill State Park, southern Indiana, marks a rare place where a mostly underground stream briefly comes into view before disappearing again. Part of a larger karst system, the water begins as Mosquito Creek, sinks into Upper Twin Cave, emerges at Twin Caves, then passes through Lower Twin Cave, Bronson Cave, the Shawnee Cave System, and finally reaches the Donaldson Cave entrance as a short tributary of Mill Creek. Seasonal boat tours enter Upper Twin Cave for about 500 feet, but much of the system is privately owned. The caves were also closed for several years because white-nose syndrome devastated regional bat populations.