A coral reef looks like architecture but behaves like a city under perpetual renovation. Each polyp lays down a thread of limestone, then its descendants build atop the abandoned skeletons, so the visible reef is mostly the accumulated past of countless tiny lives. When a heat wave bleaches the surface, the loss is real, yet the structure beneath remains a record—an archive of conditions stretching back centuries. Scientists read these layers the way others read tree rings, extracting from the stone a diary the ocean never meant to keep.