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The team journey around the great estuaries of Britain where 20 million people live, and a dazzling variety of animals thrive. Nick Crane explores the wealth of wildlife and industry that are attracted to the Firth of Forth, the mighty estuary that feeds Edinburgh, and must answer a deceptively tricky question - why is the sea salty? Nick also investigates a remarkable natural phenomenon discovered accidentally on this coast in 1834. Miranda Krestovnikoff witnesses the extraordinary transformation that salmon must make to their bodies to avoid death by dehydration as they migrate from freshwater to saltwater, and learns how Scottish fish farmers uncovered the secret of managing salmon in captivity? Tessa Dunlop reveals how the Victorian zeal for cleanliness turned the Thames into a giant self-flushing toilet bowl. Mark Horton discovers the struggle to build a rail tunnel deep under the Severn estuary between England and Wales, a challenge that was finally accomplished in 1886.