The Lost Sea is the largest non-subglacial underground lake in the United States and the second largest in the world. It is located inside the Craighead Caverns, a large cave system located in the foothills of the Great Smoky Mountains, between Sweetwater and Madisonville, Tennessee, USA. It was discovered by a 13-year-old boy in the early 20th century.
The visible surface of the lake is 240 meters long and 67 meters wide (1.8 hectares) at its normal "full" capacity. In the 1970s, divers explored several rooms completely filled with water, without reaching the end of the cave.
For many years, the Lost Sea was considered the largest underground lake in the world and is still recognized as the second largest non-subglacial underground lake in the world after the Dragon's Breath Cave in Namibia.
Today, visitors can take a boat tour of the underground lake. In times of extreme drought (such as 2007-08), the lake recedes significantly. According to experts, in 2007-2008, the water level in the lake receded 28 feet below its normal level. At that time, visitors were able to see a much larger cavern on the lake's surface.