A coral reef is a vast living structure built over thousands of years by colonies of tiny animals whose limestone skeletons accumulate into the most biodiverse habitat in the sea — a system that covers a tiny fraction of the ocean floor yet shelters perhaps a quarter of all marine species, shields coastlines, and feeds hundreds of millions, and that warming and acidifying seas have now made one of the most threatened ecosystems on Earth.
A coral reef looks like rock and lives like a city. The stone is the work of animals so small that a single one could sit on a fingernail. Over thousands of years their limestone skeletons pile up into vast underwater ridges, and into them crowds a startling share of all the life in the sea. Reefs cover less than one percent of the ocean floor, yet about a quarter of all marine species depend on them. And now the warming water that built them is starting to kill them.