Oceans come and oceans go. Once they cut a deal with the corals, but they didn't mean forever. Deals come apart and circumstances change. The earth gets warmer and corals get a dose of stress. There is an important lesson to be learned in the long-term view, from the coral fossil record.
Corals are good predictors of mass extinction events.
The corals nearly died out with the dinosaurs when a comet slammed into the earth some 65 million years ago. The ancient Greeks believed that comets come when the gods are angry. The medieval view had it that comets were signs of a ruined world that has fallen into sin. I'm more inclined to think of comets as something for which neither the gods nor I am personally re- sponsible, but rather a comet is factor X entering the evolutionary equation. Things trundle along, one thing leads to the next, until one Lay, sullenly, it doesn't.