Anna Turns investigates what over 30 years of post mortems on dolphins, porpoises, and whales has revealed about the state of the seas. The Cetacean Strandings Investigation Programme in England and Wales, and the Scottish Marine Animal Strandings Scheme, have carried out thousands of autopsies. Anna goes into the pathology lab with Rob Deaville from ZSL as he examines a Harbour Porpoise for clues about how it died, and how it lived. As Anna finds out from toxicologist Dr Rosie Williams and veterinary pathologist Dr Andrew Brownlow, evidence from post mortems shows animals' ability to survive and breed is threatened by pollution from long banned but peristent chemicals, known as PCBs. To find out how these chemicals could still be leaching into the environment Anna travels to the Thames Estuary with Professor of Environmental Geochemistry Kate Spencer. Producer for BBC Audio in Bristol : Sarah Swaddling