This world-class aquarium is the most enduring and impressive achievement of EXPO '98. Marketed as the second-biggest aquarium in the world (the largest is in Osaka, Japan), it's in a stone-and-glass building whose centerpiece is a 5-million-liter (1.3-million-gal.) holding tank. Its waters consist of four distinct ecosystems that replicate the Atlantic, Pacific, Indian, and Antarctic oceans. Each is supplemented with aboveground portions on which birds, amphibians, and reptiles flourish. Look for otters in the Pacific waters, penguins in the Antarctic section, trees and flowers that might remind you of Polynesia in the Indian Ocean division, and puffins, terns, and seagulls in the Atlantic subdivision. Don't underestimate the national pride associated with this huge facility: Most Portuguese view it as a latter-day reminder of their former mastery of the seas.