Ruby Falls is an underground waterfall in Ruby Falls Cave on Lookout Mountain in Chattanooga, Tennessee, long celebrated as one of the mountain’s best-known attractions. Formed in limestone deposited roughly 300 to 360 million years ago, the cave developed as uplift and groundwater created passages, chambers, and a vertical dome-pit where the falls descend. Leo Lambert discovered the waterfall in 1928 while seeking to reopen nearby Lookout Mountain Cave and later named it for his wife, Ruby. Opened to the public in 1930, it became an early commercial cave attraction and gained national fame through roadside advertising, later earning National Historic Landmark status for both geological significance and tourism history.