Located on the 5th and 6th floors of Tokyo Skytree, the Sumida Aquarium is home to over 10,000 sea creatures. At its centre is a 350,000-litre tank, considered the largest open indoor tank in Japan, which is home to dozens of penguins and several fur seals. The aquarium is famous for its complete use of artificial seawater in all its tanks, making it a paragon of sustainability.