Episode 4A - "The Underworld": Pepper teaches C.T. the difference between air pressure and water pressure along with everything else he needs to know about scuba diving. After two days of training, they go on their first dive and practice clearing their mask buddy breathing. Victor, Terry and Quiche board the Mimi and go an a dive with Pepper and C.T to find out if there's a channel in the reef for Mimi to pass through. Meanwhile, Captain Granville and Quiche go snorkeling and discover an ancient Mayan artifact. They all work together to bring the artifact aboard the Mimi. Expedition 4B - "The Incredible Shrinking Head": Hosted by Ben Affleck. Ben visits the North American Hyperbaric Center on City Island in New York City. He meets with Dr. Glenn Butler, a diving expert at the center, to learn about what air and water pressure is all about.