The Season 4 premiere travels to Manuk, a small volcanic island in Indonesia, to learn why its waters are teeming with thousands of venomous sea snakes. Also: a visit to a remote atoll .. show full overview

Bird observes bull sharks off the Mexican coast; and visits NASA's Neutral Buoyancy Lab in Houston, where astronauts train for space walking in a pool that's about half the size of a football field.

The endangered Hawaiian monk seal; injured military veterans who learn to scuba dive as part of their physical therapy and healing; the Great Annual Fish Count off the coast of Massachusetts.

A visit to the Florida Keys examines artificial reefs made from ships that were intentionally sunk. Also: a freediving lesson in Bonaire from world-champion freediver Karol Meyer.

Efforts to save the sturgeon in New York's St. Lawrence River by rebuilding their spawning habitat are examined. Also: an underwater farm in Key Largo, Fla., where coral is grown.

A visit to the Mystic Aquarium in Connecticut spotlights beluga whales, including how they're trained and cared for. Also: a 400-foot-deep blue hole located on Andros Island in the Bahamas; a playful pod of spinner dolphins off the Hawaiian coast.

Aquarius Reef Base, the world's only undersea lab, is visited. Also: the world of skates and rays.

An underwater cave system in a remote part of Andros Island in the Bahamas is explored. Also: the world of sponges.

How whitetip reef sharks hunt. Also: the kelp forests off the coast of Catalina Island; and shipwrecks on the St. Lawrence River.

The Queen Anne's Revenge, which was the pirate Blackbeard's ship, off the coast of North Carolina; manta rays that feed off the Kona coast in Hawaii, including one that has been coming to the feeding site since 1979.

The Season 4 finale spotlights underwater invasive species and their effect on the environment; and a special submarine that dives 1000 feet below the surface.