Forested reef, Blue Hole National Monument, Belize

© Tom Till/Alam

The other great barrier reef. Belize Barrier Reef

One of the greatest marvels of the marine world, the Belize Barrier Reef runs 190 miles along the Central American country's Caribbean coast. This mangrove forest is found on Lighthouse Reef, one of three major atolls (above-water sections of reef that form ringlike, often lush island chains) in the reef system. It's the most-visited atoll for one reason: Near its centre is the Great Blue Hole, a protected marine sinkhole that's 1,043ft (318m) across, 407ft (124ft) deep, made famous when oceanographer Jacques Cousteau charted its depths in 1971.

Belize's most popular tourist attraction, and one of the most biodiverse places in the world, the reef system includes more than 400 cays or islands, mangrove forests, coastal lagoons and estuaries, and is the only place in the Western Hemisphere to find coral atolls.

It is part of the larger Mesoamerican Barrier Reef System that runs from Mexico's Yucatan Peninsula to Honduras, the second-largest reef in the world, after the Great Barrier Reef in Australia. This Unesco World Heritage Site has remained relatively healthy, while many of the planet's other reefs are threatened by climate change.