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Vladimir Putin has dived to the bottom of the world's deepest lake in Siberia, aboard a mini-submarine.

The Russian prime minister descended 1,400m (4,600ft) in a four-and-a-half hour mission to inspect crystals containing natural gas.

Mr Putin said it was a very special feeling and he had not seen anything like it before.

The mission is likely to add to the 56-year-old's carefully cultivated image as a man of action.

However, as he emerged after the dive, Mr Putin quickly played down reporters' suggestions that his next challenge would be to travel in space.

"There is enough work on earth," he said.

'Plankton soup'

Lake Baikal in Siberia is a UN world heritage site. The largely untouched lake bed contains deposits of clathrate hydrate - crystals packed with natural gas.

Experts say the reserves of methane could rival some of the largest deposits in the world, although mineral extraction is banned.

"What I saw impressed me," the Russian leader said. "With my own eyes I could see how Baikal is, in all its grandeur, in all its greatness.

Mr Putin yesterday tagged a whale called Dasha

But Mr Putin expressed some surprise at how murky the water was. And while he realised it was clean from an "ecological point of view", he described it more as a "plankton soup".

Mr Putin arrived for his dive a day after clipping a satellite transmitter onto a Beluga whale on Chkalov island in Russia's far east.

These acts are the latest in a series that have helped create something of a macho image for the former KGB spy.

While president, Mr Putin was famously photographed on holiday angling, stripped to the waist and wearing a pair of combat trousers.

As well as being a judo black belt, he has also taken to the sky in a fighter jet, shot a Siberian tiger with a tranquilliser gun, and been pictured astride a Mongolian horse.