The reef grew upward from the deep substrate, colonizing one horizontal shelf at a time as it reached toward the light. By the third survey the entire water column was filled with living coral. But the thermal anomaly that summer hit the shallowest layer hardest — the shelf closest to the surface, where the water was warmest and the coral youngest. That topmost row bleached completely while the deeper shelves, insulated by cooler water, survived. The reef recovered the following season.