From the World Heritage inscription:
The coastal area of Belize is an outstanding natural system consisting of the largest barrier reef in the northern hemisphere, offshore atolls, several hundred sand cays, mangrove forests, coastal lagoons and estuaries. The system’s seven sites illustrate the evolutionary history of reef development and are a significant habitat for threatened species.
The reef extends from the border with Mexico to the north, to near the Guatemalan border to the south. The Belize submarine shelf and its barrier reef, represent the world second largest reef system and the largest reef complex in the Atlantic-Caribbean area. Outside the barrier, there are three large atolls: Turneffe Islands, Lighthouse Reef and Glover’s Reef.
The reef system in Belize is second only in size to the Great Barrier Reef in Australia. Unlike the Great Barrier Reef, however, the reef in Belize is much more accessible. The Great Barrier Reef cannot be seen from shore and is an hour boat ride away. The Belize reef can be see from shore on any number of cayes in the area and going to a snorkel or dive spot doesn’t take that much time.
To get to the reef, just take a water taxi from Belize City to either Caye Caulker or San Pedro. From there you can see the reefs and easily take a day snorkel or dive trip.
View my complete list of UNESCO World Heritage Sites.