The six-gill shark is one of the largest and most mysterious sharks in the ocean. They've been around since prehistoric times and can grow up to 20 feet in length. And because they live in the darkest depths of tropical oceans, few humans have encountered them. Shark Diver is the only company in the world to use research-grade mini-subs to bring tourists down 2000 feet into the Cayman Trench, where they'll see some of the planet's rarest marine life and come face to face with the steely eyes of this giant monster of the deep. It's lured to the sub's window by a tasty pig's head.

For more information: Deep Sub Shark Adventures

Near…Far…Wherever you are, the lure of the world's most famous shipwreck continues to entice more than just the imaginations of James Cameron's devotees. A select few expeditions depart the Canadian mainland each summer, setting out over the choppy North Atlantic aboard sturdy research vessels outfitted with Russian MIR submersibles, which dive the two and a half miles down to reach the "unsinkable" ship's remains. Only a few dozen people get to see the Titanic each year, which means your bragging rights at the bar are almost assured.

For more information: Titanic Dive. St. John's

For those who love choosing their dinner from the lobster tank, the swimming buffet just beyond the clear acrylic at the Ithaa Undersea Restaurant will leave you drooling. This unique eatery sits 16 feet below the tropical water's surface, right next to a vibrant coral garden. Though not much larger than a subway car, the unobstructed dome allows views of an entire ecosystem to accompany the cuisine, which is a fusion of Western and local flavors, largely based around seafood. And while you can't just point to your right and say "I'll have that one," a generous tip to the busboy might induce him to fetch a spear gun.

For more information: Ithaa Undersea Restaurant

Slated to open sometime in 2009 or 2010, the Poseidon will likely be the world's first underwater resort, where guests will descend via elevator to individual suites on the ocean floor. On-site activities will include mini-sub pilot lessons, submarine excursions into the ocean's depths, scuba diving and good old-fashioned beach bumming. The resort's developer even plans to build a luxury guest cabin that will be perched on a wall 1000 feet below the surface, accessed by a private sub. Just be sure not to forget your keys at the restaurant. Future resorts are planned for sites in the Mediterranean, Caribbean, Red Sea and Indian Ocean.

For more information: Poseidon Undersea Resort

The pioneer in underwater lodging is more motel than hotel, but it's also the only undersea dwelling in the United States. Guests must be certified scuba divers to reach the lodge, but once down they can relax in the aquatic-themed interior (think: sea scenes printed on linens and pillows shaped like fish). The lodge's "Mer-Chef" dives down to cook meals each night, and can even cater weddings, should you wish to pledge your everlasting love in every conceivable environment.

For more information: Jules' Undersea Lodge

Though construction hasn't yet begun, financing is well underway for these aquatic super-complexes, which very much resemble space stations from sci-fi novels. Each will be anchored close to a land-based megatower, which will connect to the floating cities by way of tunnels. These won't just be resorts but "Oceanic Settlements", where people will live, play and, presumably, fish. Though Dubai and Qingdao are the first slated to open, there are talks of additional locations in London, Monaco, Munich and New York—all cities where the price of land could drive people to the apartments Hydropolis plans to sell out at sea.

For more information: Crescent Hydropolis Resorts

As if a "Unite Me-Crystal Ritual" treatment, complete with local virgin coconut oil wood massage, wasn't relaxing enough, this new resort has taken the spa experience and plunged it underwater, with two double treatment rooms suspended right amidst the corals. Peering out through the windows, as trained hands knead away whatever possible stress remains in your body, be sure not to hum "Octopus' Garden" too loud. You might scare the fish.

For more information: Huvafen Fushi Resort

Why pay guides and tour companies to take you on submarines, when you can just pony up and buy your own? The SEAmagine models are the exact same types of deep-sea submersibles used to explore shipwrecks and research marine life, and can descend hundreds of feet below the surface. You'll probably have to buy an appropriate adventure-class yacht to lower the sub into place, but what's another multimillion dollar vessel on the credit card?

For more information: SEAmagine Submersibles

U.S. Submarines has done for personal subs what Gulfstream did for airplanes—married the styling and comfort of a luxury yacht with the versatility and technology of military-grade equipment. The company's vessels range from two- and three-passenger personal submersible pleasure craft to sunken pleasure palaces that can stay under for weeks at a time with staterooms, kitchens, scuba escape hatches and detachable mini-subs. It's the ultimate in discretion and big-toy luxury, and when you buy one, you purchase the rare opportunity to yell "Dive! Dive! Schnell!" at your bewildered crew. The only downside is that they come with neither periscopes nor torpedoes.

For more information: U.S. Submarines

The biggest long shot in the undersea resort game is the aptly named Undersea Resort. Backed by a Florida plastics company that intends to build floating (not moving) cruise ships with underwater viewing areas, these will be permanently moored offshore or in harbors. They promise all the amenities of cruising—theaters, spas, water slides—plus views of sea life. So far, they've received no orders—proof positive that underwater resorts are still quite speculative.

For more information: Undersea Resort