How do coral polyps build a reef over time?

Coral reefs are built by colonies of tiny animals called polyps, each of which secretes a hard calcium carbonate skeleton. When polyps die, new polyps grow on top of the old skeleton, gradually forming reef structure. Reefs grow best in warm, clear, shallow water where sunlight reaches the algae living inside many corals. Over thousands of years, this process can create enormous reef systems such as the Great Barrier Reef. Reef growth is slow and vulnerable, so warming and acidification can weaken the ability of corals to build skeletons.