The National Marine Aquarium in Plymouth holds the title for the largest aquarium in the UK as of 2026. This massive facility is home to over 5,000 marine animals and features the deepest tank in the country, the "Atlantic Ocean" exhibit, which holds a staggering 2.5 million liters of water. This specific tank is so large that it contains a life-sized replica of a WWII seaplane and is home to sand tiger sharks, nurse sharks, and a green turtle. Another major contender is SEA LIFE London, located on the South Bank, which is the largest aquarium in the capital and famous for its "Shark Walk" where guests walk over a glass floor above a shark-filled tank. While the National Marine Aquarium is the leader in total water volume and conservation research, other sites like Deep Sea World in Scotland (featuring one of the world's longest underwater tunnels) and The Deep in Hull (an "international submarium") offer unique experiences. For a 2026 visitor, Plymouth's National Marine Aquarium is the "gold standard" for scale, providing three distinct zones that take you from the local shores of Devon to the tropical reefs of the Great Barrier Reef.