One Planet, One Ocean is the main attraction at the Lisbon Oceanarium. This huge aquarium contains 5 million liters of seawater. This immense volume of water could fill over 2 Olympic-sized swimming pools, or it's equivalent to about 2,000 average-sized bathtubs. The aquarium is artificially temperature controlled to maintain ocean temperature and salinity levels for its residents which include penguins, puffins, colorful butterflyfish, unicornfish, axolotls, otters and many more