In the final programme of the series, explorer Paul Rose, marine biologist Tooni Mahto and underwater archaeologist Frank Pope explore our busy southern shores. As one of the world's busiest shipping lanes, our southern seas are littered with wrecks. Outside Poole harbour, the team join an archaeological excavation of a mysterious 17th century ship. Bearing incredibly ornate carvings of a merman, Frank reveals this is one of the most important underwater discoveries since the Mary Rose. Paul joins the dive team working on the wreck to try to discover where this ship came from. But it's a race against time as the timbers are being destroyed; Tooni wants to find out why. Frank and Tooni examine how we balance the needs of marine life with those of people when the two come into conflict. Studland Bay in Dorset is home to a thriving population of spiny seahorses living amongst the rare eelgrass beds. Only identified as breeding in the area seven years ago, the intensive use of the bay, boat moorings and pleasure boats' anchors are threatening the seahorses fragile eelgrass habitat. Paul dives two wrecks in the search of the mighty conger eel to find out if, when he comes face to face with one, these incredible beasts will live up to their fearsome reputation. As the tourists hit the roller coaster and dodgems on Brighton pier, they know little of the wonder beneath them. Tooni dives underneath Brighton pier to reveal one of the best places on the south coast to see a great variety of our native marine life. Out in Lyme Bay, Frank dives the wreck of an experimental WWII submarine.