How do tiny corals build a whole reef?

One little cup at a time. A coral polyp is a soft animal smaller than a pea. It pulls minerals out of the seawater and turns them into limestone, the same kind of rock as chalk, and builds a little cup of it underneath itself. New polyps grow beside it, each building its own cup, until one coral is home to thousands of them.

When a polyp dies, its cup stays, and new polyps build on top. Layer over layer, over thousands of years, that slow building made the Great Barrier Reef, more than fourteen hundred miles long. It’s the largest thing living creatures have ever made, and the polyps that built it have no eyes.