The breathtaking centerpiece of Boston's beloved aquarium is the colossal 200,000-gallon salt-water replica of a Caribbean coral reef. The cylindrical tank, 40ft in diameter and three stories tall, is alive with moray eels, stingrays, gigantic sea turtles and menacing sharks. On a smaller scale, a touch tank exhibit lets children stick their hands into the cold water of a tidal basin and get up close and personal with starfish, sea urchins and hermit crabs. The huge indoor penguin exhibit (constructed so almost all of the balconies overlook it) is a hoot. If the lines are too long, peek at the playful inhabitants of the outdoor seal enclosure instead. The IMAX theatre offers state-of-the-art 3D glasses to put viewers in the middle of the action. From April to October, the Aquarium runs a naturalist-narrated whale watch boat trip, which visits one of the largest whale feeding grounds in the world.