BILLIONAIRES are going to ever greater depths to outdo each other: they are competing to have the biggest private submarines that money can buy.

Paul Allen, co-founder with Bill Gates of the Microsoft empire, recently bought a bright yellow submarine capable of taking 10 passengers. The craft is said to be docked, James Bond-style, inside Octopus, his 416ft vessel, claimed to be the world’s largest yacht.

Last week Paul Moorhouse, a Plymouth-based submarine designer, said that two oil billionaires in the Emirates now own private submarines offering pressurised overnight accommodation, and that an additional “seven or eight extremely wealthy people” have invested in more modest two-man subs.

“You have to be weird to want one,” he declared. “They cost at least £10m to build and £100,000 a year to maintain.”

Roman Abramovich, the Russian billionaire who owns Chelsea football club and four super-yachts, has a two-man “run-around” sub, which sits alongside his helicopter on the 340ft Pelorus.

A source last week implied that he may want to trade up: “If other people have got bigger ones, he will have to be told that he’s behind the times.”

The ocean depths are seen as an exclusive playground for the super-rich and one entrepreneur is preparing to build the world’s first submarine cruise ship.

The vessel, to be named Poseidon, is aimed at the booming market for luxurious but extreme adventure and will be the first commercial vessel to provide cruises to the bottom of the sea.

Costing £100m, the 286ft ship is designed to perform as well on the surface as it does submerged. The intention is to enable tourists to hop from port to port but also to spend several days at depths of 1,000ft, observing wonders such as the Great Barrier Reef and undersea formations off Caribbean and Hawaiian islands.

It is the brainchild of Bruce Jones, a submarine entrepreneur from Fort Lauderdale, Florida, who believes that deep ocean tourism rivals space as a new frontier for holidaymakers. Three multi-millionaires have already paid fortunes to fly on space missions.

Jones, a member of the American Bureau of Shipping’s committee on underwater systems, has designed the Poseidon and is raising finance for its construction. He believes it can be in service within three years.

The design envisages accommodation for 70 passengers in luxury staterooms costing upwards of £1,300 a day. Part surface ship, part submarine, the Poseidon will have large acrylic windows capable of withstanding the pressures of extreme depths while giving floodlit views of the undersea world. The mother vessel will also carry a smaller submersible for close-up exploration of reefs and wrecks.

Jones is confident that there will be almost unlimited demand. “The idea of this kind of experience captures people’s imagination,” he said. “There are millions of intelligent high-end tourists in the world who are fascinated by the idea of underwater travel. We will be able to accommodate only a few thousand a year and our research shows massive interest.”

In the Bahamas he is already developing the Poseidon underwater resort, the first submerged hotel. Planning and finance are in place and Jones hopes the 22-room facility will open next December.

Since status symbols such as mega-yachts have become more common, billionaires are vying to find novel and extreme ways to outdo each other. So as well as submarines, the super-rich are seeking unusual planes. Larry Ellison, boss of the computer compay Oracle, has his own jet fighter.

The most distinctive display has come from Gates, who is Ellison’s arch-rival. After giving billions to charity, he can probably claim the title of the world’s greatest philanthropist.

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