Andros Barrier Reef, Bahamas
Anonymity has been kind to the Andros Barrier Reef. The world's third-largest reef, at more than 170 miles long, it sits less than a mile off the eastern shore of Andros Island, which means it's a mere hour's plane ride, followed by a quick boat trip, from Miami. But thanks to complex land-ownership laws that have kept Andros Island free of big resorts, the reef has been left largely unexplored. "The local population is so small that the pressure on the reef is minimal," says Sylvia Earle, oceanographer and National Geographic explorer-in-residence. "It's undisturbed and it's crowded with sea life."

But what makes diving here so memorable is the underwater topography: Just offshore, the reef plummets into the Tongue of the Ocean, a 6,000-foot-deep trench where the U.S. Navy test-drives subs. One of my favorite dives is Over the Wall, the Corbett's Couloir of the scuba world. Diving it last summer, I saw reef sharks and spinner dolphins hovering at the edge of the abyss. I went into a ten-story free fall until the dark water beneath me warned that I'd reached my depth limit.

STAY: In one of 17 beachside cottages at the Small Hope Bay Lodge, a family-run resort that started leading dives to Andros Barrier Reef in 1960. $2,083 per week, including meals, basic diving equipment, and three single-tank dives per day; smallhope.com