The Western Hemisphere's four atolls (three in Belize, one in Mexico) are by nature isolated, miles offshore, making them less subject to pollution than coastal reefs. Thirty miles long, Chinchorro Bank (protected since 1996) is the Caribbean's largest atoll, a ring of reef and coral islands surrounding a lagoon. It lies 19 miles off Costa Maya and Mahahual in the southern Yucatán. Weather permitting, snorkelers and divers make the 90-minute run to the atoll to see Mexico's richest reef ecosystem, including elkhorn growing right to the surface, black coral "trees" as shallow as 30 feet, genius-sized brain corals, and barrel sponges as big as the spinning tea cups at Disney World. There are also dozens of wrecks, and to prevent looting, only snorkeling is allowed on them.