Six hundred miles from the nearest ocean, eastern Missouri isn't an obvious choice for scuba diving. But the rural hamlet of Bonne Terre offers world-class diving just the same – in a former lead mine 60 miles due south of St. Louis. From the 1860s until 1961, the Bonne Terre Mine became the largest lead mine in the world, yielding millions of tons of lead ore. After it shuttered, a billion gallons of groundwater flooded the 80-square-mile labyrinth, creating the world's largest man-made underground lake and turning the former mine into the ultimate underwater playground – a cross between cavern and wreck diving, with eerie tunnels crusty with antiquated equipment hidden in the shadows.
More than 50 charted trails are in the mine, threading narrow tunnels littered with old magazines, rock drills, and half-filled ore carts that still sit where the miners dropped them 50 years ago, like the last vestiges of a working man's Atlantis. The most popular paths are overhung with stadium lighting, which illuminates the water's 150 feet of visibility. Wetsuits are highly recommended – the water stays a brisk 60 degrees year-round. To avoid getting lost in this industrial-size ant farm, certified divers must go in the water with guides from the on-site dive center [packages from $210; 2dive.com], which is open on weekends and provides gear and scuba classes, as well as accommodations in a historic railroad depot restored as a bed-and-breakfast.