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... (Wikipedia)