How do tiny animals build a whole reef?

A reef is built a few millimetres at a time, by animals that never meet the finished building. Each coral polyp deposits calcium carbonate, a hard mineral, as a skeleton beneath and around itself, which pushes the living top of the colony up and outward. When polyps die their skeletons stay behind, new polyps settle on top, and generation after generation the layers pile up into a reef held together by that mineral.

Corals are not the only builders: coralline algae deposit limestone in sheets over the reef and form a tough crust on the side that faces the open ocean, where waves hit hardest. Meanwhile other creatures take the reef apart. Waves, parrotfish, sea urchins and sponges break skeletons into fragments that fill gaps in the reef or settle as the sand of the lagoon. Parrotfish bite off chunks of algae-coated coral, digest the algae and excrete the coral as fine sand, so some of a reef's sand has passed through a fish.

Growth is slow but steady. Healthy tropical reefs spread sideways about 1 to 3 centimetres a year and rise 1 to 25 centimetres a year, but only in water shallower than about 150 metres, because their algae partners need light.

That is also why most living reefs are young. Most were formed after the last ice age, when melting ice raised the sea and flooded the edges of the continents, and most are less than 10,000 years old. The reefs grew upward as the water rose, and the ones that rose too slowly drowned in water too dark to live in.