A coral reef is not made of rock - it is made of tiny living animals called coral polyps, which build hard skeletons around their soft bodies. Over hundreds and thousands of years, millions of these skeletons pile up to create a huge, colourful underwater structure. The Belize reef has been growing like this for thousands of years.
The reef sits very close to the shore in some places, which means you can snorkel right from the beach and see astonishing colours beneath you. Parrotfish, angelfish, eagle rays and sea turtles glide through the coral gardens. Whale sharks - the biggest fish in the ocean - visit the reef too, though they are completely gentle and feed only on tiny creatures called plankton.
The whole reef system is part of a UNESCO World Heritage Site, which means countries around the world have agreed it is one of Earth's most precious places and needs to be looked after. Belize works hard to protect the reef by limiting fishing in some areas and teaching visitors how to explore it without causing harm.
If you visit Belize, many local guides - some from fishing families who have lived on the coast for generations - will take you out on a small boat to snorkel the reef. They know exactly where the best coral gardens and friendliest sea turtles are.