The Barrier Reef lies about half a mile off the winward side of the island. It is the longest barrier reef in the Western Hemisphere and the second longest in the world.
To the east of the Barrier Reef are three separate atoll reefs. There is also a fourth atoll reef, Banco Chinchorro, just to the north in Mexican waters, which will be of particular interest to wreck divers.
The three Belize atoll reefs are formed on two tiers of submarine ridges: Turneffe and Glover's on one ridge and Lighthouse on a separate ridge farther to the east. This accounts for their similar outlines and NE-SW orientations. Deep marine trenches separate the two ridges.
Flying south into Belize, you can see the reef as an unbroken chain of white surf running along the Caribbean coast of the Yucatan Peninsula and continuing south almost the whole length of the country to the Ranguana and Sapodilla Cayes.
For extensive information on the Barrier Reef, click here: http://AmbergrisCaye.com/pages/town/barrier.html