Forget the tranquil waters teeming with colorful sea life off white, sandy beaches. Some people like a different kind of diving experience that takes them where few men (or women) have gone before. We're talking mangroves, fjords, beneath the ice and other watery netherworlds. If you don't mind some extreme conditions like frigid cold and maybe even a crocodile or two, this is your chance to see some of the planet's murkier (but just as spectacular) underwater realms — extreme otherworldly beauty like undersea mountains, shipwrecks, kelp forests and much, much more.