Any child with a working knowledge of Raffi knows all about that friendly “baby beluga,” swimming so happily and freely in the sea. But there are few opportunities in the country to come face-to-face with these playful white whales that form the centerpiece of Mystic’s second-most visited attraction (after the seaport). Three 1,000-pound belugas inhabit the one of the largest such tanks in the U.S., which has both underwater viewing areas into tunnels and ice caves, and above-water ledges from which you can watch the whales spout and feed. Other exhibits include penguins, jellyfish and sea lions, who regale guests with their antics at regular shows.