On a southern Mexican beach, more than 300,000 olive ridley turtles have performed one of nature's most ancient rituals — a mass nesting that speaks to a species refined over a hundred million years of survival. Yet the sand into which they have laid their eggs is warmer and drier than it once was, shaped by a climate that is shifting faster than evolution can answer. The spectacle of arrival is intact; it is what comes next, in the quiet beneath the surface, that now hangs in the balance.