Ruby Falls is in a cavern under Lookout Mountain in Chattanooga TN. It was a huge surprise for me and we loved it. We did Rock City and Ruby Falls on the same day. This is the largest waterfall underground in a cave in the US (and world?). Its 145 foot tall and underground about 1100 feet. Very impressive cave and waterfall. The guy that discovered it was drilling a shaft to get to the cave under Lookout mountain when they cut through another crevice under the mountain. He crawled into a 2 foot high by 4 feet wide crevice for 7 hours until he could stand up, He spent a total of 17 hours exploring this new cavern. Very impressive story.