A hundred years back, cuss-tough miners employed by the St. Joseph Lead Company used dynamite to hollow out the massive mine, located an hour's drive south of St. Louis. It has since been abandoned, but the miners' world remains - lunch pails, picks, and rock steps scuffed by boot marks - entombed by the silent weight of a billion gallons of limestone-filtered water.

CUYANA MINE PITS, MINNESOTA
Minnesota School of Diving
, (800) 657-2822, www.mndiving.com
Central Minnesota's Brainerd Lakes area is best known for fishing and golf, but the astute diver will also notice, well, lots and lots of lakes. These lakes are pocked with interesting dives, and, an added bonus, potentially shocking visibility (as good as 70 feet). The area highlight is any of the 55 now-flooded open-pit iron mines, many with sheer walls (some mines descend past 500 feet) and submerged forested meadows, with the occasional flyover of a huge northern pike.

SUWANNEE RIVER, FLORIDA
Aquaspeleo
, (386) 776-1191, www.aquaspeleo.com
Of the 200-plus springs along northern Florida's Suwannee River, several dozen are diveable, and they're spectacular - bath-water warm and mirror-clear (100-foot-plus visibility). Great places to snorkel, too. Start on the upper river and dive Troy Springs, where, 70 feet down, you'll see the wooden ribs of a Civil War steamer.

THE BLUE HOLE, SANTA ROSA, NEW MEXICO
Santa Rosa Dive Center
, (505) 472-3370
Sixty feet across and 80 feet deep, the Blue Hole's waters match Santa Rosa's skies; water so clear there's no place for anything to hide, including the goldfish, carp, bass, trout, catfish, and crawdads dropped there by area divers. A favorite pastime is gazing up, from 60 feet down, at night. "You can see the stars, and watch the moon go over," says diver Rudy Salazar. "It's incredible."