Together with another two NASA crew members, the astronauts are spending 10 days under water to prepare for outer space flights, and say such extreme environment training could come in handy on a mission to Mars.

Their temporary inner space home is Aquarius, a coral-encrusted cylinder touted as the world's only working undersea habitat.

Located 5.5 kilometers (3.5 miles) off Key Largo, Florida, at a depth of 27 meters (60 feet), the research station looks a little like a yellow submarine resting on the ocean floor.

Massive groupers and colorful tropical fish nibbled on the underwater laboratory, which is about the size of a rail car.

A nurse shark rested on the ocean floor, seemingly unperturbed by the presence of John Herrington and Doug Wheelock who assembled a PVC structure on the sandy ocean floor.

"It's a good place to try things out before we use them in space," said Herrington, the mission's commander.

Herrington says there are a lot of similarities with conditions in space.

"It's very hectic, we have to get a certain amount done in a certain period of time, that's identical to space life," he told AFP.

"You have to be very careful with your body position, you have to simulate and handle things under water, you can't drop them ... you have to deal with a lot of tools ... you have the same kind of hand-eye coordination needed on a space walk."

Because they are spending so long under water, the astronauts are saturated with nitrogen. Before they return to the surface they will need to undergo a 17-hour decompression in the Aquarius, with atmospheric pressure gradually reduced from ambient to that on the surface. Skipping that step could be lethal.

"If anything goes wrong we have to deal with it ourselves. We cannot go back to the surface immediately. This is what we call an extreme environment," said Wheelock.

As such, conditions are similar to those on the Moon or Mars, the aquanauts said.

But there are also major differences with a space mission.

When working outside the module, the astronauts don scuba gear with twin air tanks rather than space suits. Inside, they cannot float to the ceiling as they would in space.

But they all agree that the view, just like in space, is awesome.

On their first day, the astronauts had some graceful visitors. "There were two manta rays doing loops. They were flying in close formation, it was spectacular," said Wheelock.

"It's the perfect scenery to fall asleep to, looking out the window, looking at the fish swim by," said biomedical engineer Tara Ruttley, who is testing equipment designed to keep crews healthy on space missions.

The July 12-21 simulation -- known as NASA's Extreme Environment Mission Operations (NEEMO) -- includes research on how to counter changes in human physiology caused by microgravity.

The crew is also testing so-called silver ion technology, by wearing silver-laced shirts to assess whether this can reduce the amount of bacteria and whether such technology could be used as an antimacrobial in outer space.

They are also trying out wireless technology, which enables them to communicate with each other, and with their Houston, Texas base while diving.

The astronauts spend about four hours a day in what is known in space talk as "Extra Vehicular Activities", working on a variety of tasks outside the habitat. They return to their home under the sea through a double lock system.

Their accomodations are more cramped than those on the International Space Station, said Herrington, who has conducted a mission to the ISS and done several space walks.

They share the six bunks with two technicians from the National Oceanic and Atmospheric Administration (NOAA), the organization that runs the unique underwater lab.

The 81-tonne habitat has a shower and a toilet, a microwave, a regrigerator and computers linked to the shore base in Key Largo, about 100 kilometersmiles) south of Miami.

Air is provided through an "umbilical cord" coming from the "Life Support Buoy" moored above the Aquarius, which is equipped with compressors and generators, and signs telling fishermen to stay away.

About 14 meters (45 feet) long and three meters (nine feet) wide, Aquarius was built in 1987 and initially set in the US Virgin Islands before being redeployed off the Florida Keys in southern Florida in 1997.

It has hosted five previous teams from the National Aeronautics and Space Administration (NASA), as well as numerous scientific missions studying marine life.

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