Along the coastlines of South Korea, coral reefs that have structured marine life for generations are visibly weakening — a quiet but measurable consequence of warming seas. Divers who know these waters by memory and scientists who track them by data are arriving at the same troubling conclusion: the ocean is changing faster than its oldest inhabitants can adapt. This regional story is, in truth, a local chapter of a global reckoning with what unchecked thermal stress does to the living architecture of the sea.