Bluespring Caverns

Bluespring Caverns is a cave system located in Lawrence County, Indiana, approximately 80 miles (128 km) south of Indianapolis. The cave system is a karst and river type cave formation and drains a 15 miles² (38.8 km2) sinkhole plain. The cave contains 21 miles (34 km) of surveyed passages and is most notable for having the longest known subterranean river in the United States with approximately 3 miles (4.8 km) of navigable river.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).