The Oceanário de Lisboa is not just an aquarium, but considering its size, a world in and of itself. The Oceanarium, as it's also often referred to, is Portugal's largest indoor aquarium, holding more than one million gallons of seawater supporting the lives of 8,000 sea creatures. Four permanent exhibits represent different habitats that hold the likes of various types of birds, fish, amphibians and mammals. Here, visitors will find the likes of sea stars and coral to penguins, puffins and sea otters and everything in between. Along with a peek into life under the sea, the Oceanarium also offers a variety of activities, from guided tours to a sleepover with sharks and even a Fado show.

Visitors were blown away by how impressive the aquarium was and suggested stopping by if you need a break from the city's many historic sites. Several reviewrs in particular loved the large, central tank, and said the attraction could easily be enjoyed by all ages, and not just children. Travelers advised setting aside at least half a day to see the attraction and warned of large crowds. 

You can find the Oceanarium in the Parque das Nações, the more contemporary part of town, off of the Oriente metro stop. Admission is 16.20 euros (about $20) for adults and 10.80 euros (about $13.50) for children 4 to 12 years old. If you purchased a Lisboa Card, you'll receive a discount on admission. There is a restaurant and two gift shops located on-site. The aquarium is open from 10 a.m. to 8 p.m. daily; during the winter, it's open from 10 a.m. to 7 p.m. For more information, visit the Oceanarium's website.