Jacques Cousteau’s excellent documentary
established the Blue Hole as a magnet for divers and scientists

Amid thousands of coral heads, thriving in turquoise waters no more than 30 feet deep, the famous Blue Hole suddenly plunges to 410 feet.

The world’s largest known underwater sinkhole became an icon of Belize after 1970, when famous marine researcher Jacques Cousteau and his Calypso crew first explored the site in a mini-sub. Then in 1996, UNESCO drew more attention here by inscribing the entire Belize Barrier Reef System, including the Blue Hole, as a World Heritage site.

At the center of Lighthouse Reef Atoll, the 1000-foot circular crater, almost too precise to be natural, appears man-made. Along its shallow crest, snorkelers delight in the exquisite corals, energetic fish, colorful sponges, and graceful sea fans that sway in the clear Caribbean waters. Its rim descends to 90 feet at 60 degrees, then drops precipitously to 120 feet. At that depth, the temperature remains around 77?F all year, and visibility approaches 200 feet. Foreboding caverns perforate the ancient walls.

The Blue Hole does not sparkle with light or teem with fish. Instead it looms like an eerie netherworld. Hammer­head, black-tip, and tiger sharks prowl its depths, some­times witnessed by wide-eyed divers. But marine life avoids the hole because of its low water exchange and lack of light. The bottom is nearly devoid of oxygen.

A sinkhole occurs when rainwater erodes limestone to form a cavern, then its roof collapses. In the last ice age, the Blue Hole was one of these relatively dry systems, also called cenotes. Rich with minerals, the rainwater dripped through the porous limestone to form huge stalactites. Up to 40 feet long and six feet thick at their tops, these massive formations are the largest ever found underwater.

Eventually, the ice melted, the seas rose, and the Blue Hole flooded. Geologists descend to take core samples, which help them to understand the history of the planet. Divers, however, descend to feel the mysterious aura, wit­ness the unique decorations, and collect a trophy dive.

REEFS...A Zillion Tiny Polyps Visible from Space
Marine life depends on the Belize Barrier Reef, largest in the hemisphere, so big that astronauts can see it from space. But the huge structure is really a zillion tiny ani­mals, 1-3 mm around, called stony coral polyps. They form colo­nies of thousands, eating plankton, secreting skeletons of calcium carbonate, and growing as little as a quarter inch per year.

Symbiotic corals protect species from algae to blue marlin, through four zones: Closest to shore, a shallow flat zone remains calm and protected. It supports numerous species, including snorkelers. Stout corals in a reef crest zone sometimes peek above the surface. They serve as ramparts against waves, protecting small animals in their labyrinthine cavities. A buttress zone slopes down into deep water, with spurs of massive corals to blunt the waves, and channels be­tween them to drain the sediments. It creates an environment for large fish. In a seaward slope zone, reduced wave action below 65 feet allows many species to thrive, including sponges, sea fans, and wide-eyed wall divers.

Charles Darwin identified three reef designs: barrier, fringing, and atoll. Besides the Belize Barrier Reef - 185 miles long, 12 to 25 miles offshore - the country contains thou­sands of fringing reefs and three atolls, which resemble necklaces of coral surrounding shallow lagoons. Turneffe Islands, Lighthouse Reef, and Glover's Reef atolls, plus Chin­cho­rro Bank up in Mexico, are the four atolls of the Americas.

UNESCO inscribed the Belize Barrier Reef Reserve System as a World Heritage site, hosting 500 species of fish, 65 stony corals, 350 molluscs, assorted sponges, and crus­taceans. It harbors manatees, crocodiles, birds, and turtles.

Maya fished the reefs 2,500 years ago. Seventeenth-century pirates used them to raid Spanish ships. Nineteenth-century refugees to flee war in Yucatán. In the 21st century, they produce fish, lobster, conch, and tourists. Worldwide, reefs become damaged by pollution, sedimen­ta­tion, and overfishing. But, "Despite the many pressures the Belize Barrier Reef is under," writes UNESCO, "the commit­ment of the [Belizean] government to conservation is clearly demonstrated...."