In this episode, Steve uncovers the secrets of sharks, the world's most infamous predators. He heads out into the Atlantic ocean to dive with blue sharks, a beautiful but deadly species which can travel up to 10,000km per year looking for food. He finds out how their big pectoral fins allow them to glide gracefully through the water, and how they are now threatened by overfishing for their fins. Next up, we head into the Deadly Den on a mission to find the ultimate shark's jaw. Steve has had two shark jaws made into amazing mechanical models. First is the cookie cutter, a shark that takes perfectly circular chunks of flesh out of their prey. Steve demonstrates the cookie cutter's bite using his mechanical jaw on some fruit. Next up is the bluntnose sixgill, which uses its jaws like a hacksaw. We then head outside to see the biggest shark jaws ever to have existed - the megalodon. Using the latest CGI, we see what this true monster of the deep looked like in real life. Then it's time go in search of a very special shark - the salmon shark. These predators can hunt in waters of minus two degrees, keeping themselves warm using some amazing adaptations. To investigate, Steve heads to the Extreme Environment Lab at Portsmouth Uni, where he becomes two fish - prey Steve and salmon shark Steve. They race each other in a swimming treadmill filled with cold water, but who will win, predator or prey? Finally, Steve dives into a flooded quarry to become a great hammerhead shark. These amazing animals use super-sensing electro reception to detect prey hidden on the bottom of the ocean. Steve tries his hand at this by using an underwater metal detector to find deadly treasure hidden at the bottom of the quarry.