In the last, wrenching episode of BBC’s Blue Planet 2, there’s a distressing moment when a young Australian diver, expert in his patch of the Great Barrier Reef, admits ‘I cried in my mask’ as he swam over an ossuary of recently bleached-out coral bones. Professor Callum Roberts’s memoir of a life devoted to the study of our oceans, and in particular their coral reefs, is a ravishing, alarming account of these underwater palaces of wonder, and the existential threat they face from humanity and our warming climate.