This entertaining complex is home to more than 7,000 fish and aquatic mammals. At busy times, it seems to contain at least that many people -- in July and August, try to make this your first stop of the morning, especially on weekends. You'll want to spend at least half a day, and huge afternoon crowds can make merely getting around painfully slow. Also consider investing in a Boston CityPass; it allows you to skip the ticket line, which can be uncomfortably long, and may represent a savings on the steep admission charge. The Simons IMAX Theatre, which has its own building, hours, and admission fees, is worth planning ahead for, too. Its 85-foot-by-65-foot screen shows 3D films with digital sound that concentrate on the natural world. It's a dizzying experience.

The focal point of the main building is the four-story, 200,000-gallon Giant Ocean Tank. A four-story spiral ramp encircles the tank, which contains a replica of a Caribbean coral reef and an assortment of sea creatures that seem to coexist amazingly well. Part of the reason for the peace might be that scuba divers feed the sharks five times a day. Other exhibits show off freshwater and tropical specimens, denizens of the Amazon, sea otters, and the ecology of Boston Harbor. The floating marine mammal pavilion, Discovery, is home to performing sea lions. At the Edge of the Sea exhibit, visitors can touch the sea stars, sea urchins, and horseshoe crabs in the tide pool. The Aquarium Medical Center is especially involving -- it's a working veterinary hospital.

Naturalist-led harbor tours that teach "Science at Sea" run daily in the spring, summer, and fall. Discounts are available when you combine a visit to the aquarium with an IMAX film, harbor tour, or whale watch.