Once a dry cave system, the Blue Hole was formed after the last ice age, when the Caribbean Sea engulfed the entire area and the cave's roof collapsed. The resulting sinkhole is more than 400 feet deep, 1,000 feet in diameter--and a mainstay on divers' wish lists.
Fly 20 minutes from Belize City to San Pedro, the only real town on the resort island of Ambergris Cay, for about $110 round trip via Tropic Air (800/422-3435, tropicair.com) or Maya Island Air (800/225-6732, mayaairways.com). Or hop a ferry: Caye Caulker Water Taxis charge $16 each way for the 75-minute trip from Belize City (cayecaulkerwatertaxi.com, 011-501/226-2194).
Most places to stay are huddled around San Pedro; scout options at ambergriscaye.com. For something more removed, try the Salamander Hideaway (011-501/209-5005, salamanderbelize.com, cabanas from $130), a quiet, solar-powered resort north of town reached by a half-hour boat ride ($25 round trip).
Many agencies, including regional specialist Capricorn Leisure Corp., sell air-hotel packages to Ambergris Cay (800/426-6544, capricorn.net). Ambergris Divers (011-501/226-2634, ambergrisdivers.com) and Amigos del Mar (011-501/226-2706) offer Blue Hole day trips, departing at 5:30 a.m., returning at 5 p.m., with two hours' travel each way, for $185 plus $40 for park fees. If that's not enough time in the water, consider a live-aboard boat such as Peter Hughes Diving's Sun Dancer II: A seven-night package with meals, space for 20 passengers, and up to five dives per day starts at $1,895, plus fees of about $200 (305/669-9391, peterhughes.com).
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Due to limited sunlight and water circulation in the Hole, its limestone walls don't support all that much marine life and are rather sterile and rocky. Still, while descending, look toward the walls rather than into the hypnotizing, deep blue of the Hole's center, as the low light and lack of visual cues can be disorienting. In a cavern about 100 feet down, huge stalactites hang from the ceiling. Some are 40 feet long and more than 10 feet wide; feel free to slalom through. Most groups don't go lower than 130 feet, the maximum depth for recreational dives. Several kinds of sharks (reef, blacktip, bull, hammerhead) may appear, adding to the excitement. Attacks are extremely rare.
For generations, people believed the Blue Hole was bottomless. But in 1970 that notion was put to rest by Jacques Cousteau. Using a minisub, the famous explorer reached the bottom, at a depth of around 415 feet.