The interior houses tanks, pools and submerged tunnels designed to take the visitor through different habitats of the marine environment. The main aquarium tank holds 4,000,000 litres (1,100,000 US gal) and features a 71-metre (233 ft) underwater tunnel. Themed areas include Tropical Rivers, Lakes and Ponds, Where Land Meets the Sea, and Seas and Oceans. The aquarium contains many varieties of marine and freshwater fish, including more types of shark than anywhere else in Britain. It also keeps a variety of mammals, reptiles and amphibians.