Tour a cavernous room supported by giant rock pillars where miners dug with picks and shovels from 1864 to 1962. The temperature is 62 degrees year-round. Take a boat tour of the Billion Gallon Lake, possibly the largest subterranean lake in the world, once dived and filmed by Jacques Cousteau. Bonne Terre Mines, one of the earliest lead mines in Missouri, was the world's largest producer of lead ore. The mine consists of five levels: the lower three levels form a seventeen-mile-long illuminated underground lake; the two upper levels of the mine are used for guided walking tours, following the old mule trails. Bonne Terre Mine is a National Historic Site and a National Geographic Top 10 Adventure. Guided scuba dives are available for an additional fee. Group rates available.