Coral Sea, Australia
At the eastern edge of the Great Barrier Reef lies the Coral Sea, a two-million-square-mile submarine plateau. Visibility here can reach 200 feet, and Coral Sea divers have the increasingly rare opportunity to discover unexplored sites: Some of the coral heads were last mapped in the 1770s by Captain Cook. "Most of the dive sites don't have names yet," says Brad Doane, underwater cameraman for the BBC's Blue Planet series. "And everything is on steroids. The soft coral stalks are as big as a thigh."

GO WITH: There are only five live-aboard dive boats that cruise the Coral Sea; Cairns-based Mike Ball Dive Expeditions is the most reputable. Dive the remote Osprey and Cod Hole reefs, where eager 300-pound reef sharks show up if you smack a fist into your palm. Four-night trips from $1,400; mikeball.com