Wind Cave in the Black Hills is officially the sixth longest cave in the world, after the discovery of new passageways.
Spelunkers have been exploring the caves since the National Park Service started managing the land in 1903. They mapped and discovered 167 miles so far.
Park officials say air pressure testing indicates that only about 5-10% of the cave has been mapped, meaning there is much more to be explored.
The public cannot currently tour the caves due to elevators being replaced this summer, but the visitor center and trails above ground remain open to the public.
The Black Hills is also home to the fifth longest cave in the world, Jewel Cave.
