How do coral reefs grow into massive underwater structures?

Coral polyps are small animals related to jellyfish and sea anemones. They secrete calcium carbonate, which hardens into a skeleton that supports the colony. Many reef-building corals also live with photosynthetic algae called zooxanthellae, which provide energy and help corals thrive in clear, shallow water. When coral polyps die, new ones grow on top, gradually building large reef structures over thousands of years. The Great Barrier Reef, the world’s largest coral reef system, stretches for more than 2,300 kilometres along Australia’s northeast coast.