This complex is home to more than 15,000 fish and aquatic mammals, and 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 here, and huge afternoon crowds can make getting around painfully slow. Also consider investing in a Boston CityPass or Go Boston card; 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 for, too. Its 85-foot-by-65-foot screen shows 3-D films with digital sound that concentrate on the natural world. It's an impressive 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 twice a day. The two-floor Amazing Jellies exhibit is home to hundreds of eye-catching jellyfish. At the Edge of the Sea exhibit, visitors can touch the sea stars, sea urchins, and horseshoe crabs in the tide pool. The Medical Center is especially involving: It's a working veterinary hospital. Other exhibits show off tropical sea creatures (including clownfish -- you know, Nemo), freshwater specimens, denizens of the Amazon, marine life in the Gulf of Maine, and the ecology of Boston Harbor.

Discounts are available when you combine a visit to the aquarium with an IMAX film, a Harbor Discovery Cruise, or a whale watch.