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Triton Submarines is a Florida-based company that designs and manufactures private submarines.

Currently its deepest-diving vehicle, which was recently tested in the Bahamas, can plunge 1km (0.6 miles) down.

But it forms a prototype for a vessel that will eventually be able to make the 11km (7-mile) journey to the bottom of the Mariana Trench.

The centrepiece of the design is a glass sphere that is 15cm (6in) thick, which is being manufactured by Rayotek Scientific in San Diego.

It will have to keep the three-man crew safe from the crushing pressures of the deep - but should also provide them with a 360-degree view during the descent.

Triton's aim is a commercial one: the team plans to put tickets on sale for those who want to take the dive.

Bruce Jones, the company's chief executive, said: "You have thousands of people that climb Everest, and Richard Branson is taking tonnes of deposits for trips into near-Earth orbit.

"We think you can get $250,000 from some real adventuresome types to say they're one of a handful of people who've been to the deepest spot in the ocean."

The company hopes to have the craft ready in the next two years.