Bluespring Caverns in southern Indiana is a karst river cave with 21 miles of surveyed passages and the longest known subterranean river in the United States, including about 3 miles of navigable water. Known since the 19th century, it was first entered through a spring outlet to the White River until a 1913 dam blocked that opening. Its present entrance appeared in 1940 when a storm collapsed a pond on George Colglazier’s farm into a sinkhole. George and Eva Colglazier welcomed visitors while protecting the cave, which also shelters wildlife including the rare sightless Northern Cavefish. Today, seasonal boat tours explore part of this major Indiana cave system.