Some call it the final frontier. While humans have breached the limitations of land, air and space, the underwater world remains largely untouched.
In addition to researchers and scientists, another group has taken an interest in the underwater unknown--the mega-rich.
The race to the bottom of the sea is being led by director James Cameron and British entrepreneur Richard Branson.
This week, Cameron is launching his unprecedented mission to the bottom of the Mariana Trench in the South Pacific. The "Titanic" and "Avatar" director is hoping to make the seven-mile dive as a solo venture, which no one has ever done before.
The only pair to ever make it all the way down made the trip in 1960 and spent only 20 minutes at the site. Cameron hopes to spend six hours shooting footage of the dive for a National Geographic documentary, complete with 3D footage.
Branson unveiled a single-person submarine in April 2011 that he said would break records by exploring the five deepest sea locations of the next two years.
"More people have been to the moon than to that depth of the ocean," Bailey S. Barnard, associate editor of luxury magazine Robb Report, told ABCNews.com.
The magazine for the "ultra-affluent" has written about private submarines in the past and plans to include the vessels in an upcoming "Toys of Summer" feature.
"They're pretty darn cool and we'll continue to see more of an uptick in them as toys and vessels for exploration, as opposed to underwater homes," Barnard said.
For those who consider sports cars, yachts, and private planes old news, private submarines may be the new accessory of choice for the wealthy, ranging from about $2 million to $90 million, depending on the model.
"Most people don't have any idea what happens below the surface of the ocean," L. Bruce Jones, U.S. Submarines CEO, told ABCNews.com. "I've been doing this for 25 years and it's something that's getting more popular all the time."
In addition to U.S. Submarines, Jones is CEO of four other companies including Triton Submarines, which specializes in luxury deep-diving submersibles. Depending on the model, the subs hold two to three people, dive between about 1,000-3,000 feet and cost between $2-3 million.
The company sells about four or five subs every year, but Jones has seen an "awful lot of activity" in customer interest for the private vessels. Most of the interest has come from mega-yacht owners wanting to get a submarine for their boats that they can take out whenever they want, without having to go through an underwater tourism company.
"They can sip champagne, sit around and see things no one else has seen," Jones said. "They love it."
U.S. Submarines built one $90 million submarine that was the equivalent of an underwater yacht, complete with dining areas, kitchens and a gym. Even so, Jones does not expect submarines to become common.
"I think that they're always going to be relatively unique," Jones said. "We expect to continue to accelerate to a new place in production, but I don't think it will ever become a household item."
Ian Sheard, director of engineering for SEAmagine, a leading producer of two to three person submersibles, agrees with Jones.
"We started in tourism and then had people asking, 'Can I have one?'" Sheard said. "[Customers] want a submarine and they want to drive it themselves."
SEAmagine is currently training its latest customer to purchase one of the vessels, a sea enthusiast who plans to move his yacht and submarine all over the globe to the world's best diving spots, like Costa Rica, the Galapagos and Alaska.
So, what's the draw?
"It's exclusivity. People can't just go do it," Sheard said. "It's mind-blowing what you can see down there. The places you have the possibility of going are literally where no man has been before."
Not everyone agrees that the crafts are the new plaything of choice for the rich and famous.
"There's been an increase in the interest of submarines generated by those interested in increasing that interest," Stockton Rush, co-founder and CEO of OceanGate, told ABCNews.com. OceanGate organizes underwater expeditions, mostly for research purposes.
"It doesn't take much to double sales," Rush said, when only a handful of subs are sold each year.
"Any boat captain who lets his owner buy a submarine doesn't have control of his owner," Rush said. "A submarine and a yacht are a terrible combination for a couple of reasons."
Subs require specialized training, dedicated space on the boat due to their bulkiness and weight and special certification to travel to certain places.
"There's definitely an interest in undersea exploration and in submarines in general, but the interest is in going underwater and seeing stuff, not in owning submarines," Rush said.
Rush doesn't deny the draw of the dive, which he calls a "spiritual experience" where passengers see and hear things differently than they ever have before.
"All the sensory input you get, plus the emotional side…it's totally indescribable," he said. "Everyone gets passionate about the experience."
And for those who happen to be both passionate and wealthy, a submarine may be just the thing they're looking for.