In a white-tiled lab at London Zoo, just across the street from the giraffes, two investigators are slowly and painstakingly dissecting a porpoise.

Rescue workers recovered the stranded animal on a beach in Somerset a week before. It was maimed by brutal red gashes – from a boat’s propeller, they thought.

Investigators Rob Deaville and Matt Perkins are not so sure. Over the next two hours, they will try to uncover what killed this particular porpoise. They will also look for clues to a much bigger puzzle, one that involves all of marine life, answering questions like: what is the state of our oceans? What are the biggest threats? And what can we as humans do to help?

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Deaville and Perkins conduct post-mortems on more than a hundred porpoises, dolphins and whales a year for the Zoological Society of London. Their work has uncovered surprising threats, from long-banned chemicals lingering in the water to the devastating impact of fishing nets. But it has also revealed good news about the power of policy change and the return of endangered species.

“We use a dead body on a beach to shed light on its life, not just its death,” says Deaville, who leads the UK’s Cetacean Strandings Investigation Programme – or CSIP for short – at the Zoological Society.