A coral reef is a cathedral built by tiny animals. Each polyp, barely bigger than a pinhead, secretes its own limestone skeleton; generation after generation, they build tables, domes, arches and walls that rise dozens of yards high. Sea fans spread out against the current, barrel sponges live for centuries, and soft corals blaze with color the moment a light touches them. These landscapes, among the richest on the planet, are also among the most threatened: warming oceans are bleaching them at an unprecedented rate. Photographing them is as much about preserving their memory as showing their beauty.