Leaving the Devonian, Nigel travels forward in time to the fourth deadliest sea, the Eocene (36 million years ago), where he encounters the strange elephant-related Arsinoitherium. Diving in the Tethys Sea, Nigel encounters the primitive whale Dorudon and the crew of The Ancient Mariner uses recorded whale sounds to attract the giant predatory primitive whale Basilosaurus. In the third deadliest sea, the Pliocene (4 million years ago), Nigel encounters the strange whale Odobenocetops and dives with a adolescent megalodon shark. He later uses the rounded shark cage to dive with an adult megalodon, the largest shark of all time.

• Tim Haines

• Nigel Marven