Russian Prime Minister Vladimir Putin is diving in the depths of Lake Baikal aboard a mini-submersible today in a stunt that adds a new dimension to his carefully cultivated macho image.

The judo black belt -- who has already conquered the skies in a fighter aircraft and shot a Siberian tiger in the wild -- will descend 1,400 metres (4,600 ft) below the surface of the world's deepest lake to inspect valuable gas crystals. Hidden in the largely unexplored floor of the lake are large deposits of clathrate hydrate, crystals packed with one of Russia's most lucrative exports: natural gas.

Before locking down the hatches on the Mir-2 submersible, Putin was shown a specimen which bubbled under water.

"You can touch it. There are very few people who have held hydrates in their hand, even fewer Baikal hydrates," Robert Nigmatullin, head of the Oceanology Institute, told the prime minister. "You can set it on fire as well, it will burn."

"Let's not set it on fire today," Putin, 56, said with a smile before getting into the submersible, which then submerged below the waves of the world's oldest lake.

Spokesman Dmitry Peskov said Putin would be in contact for the entire trip.

The former KGB spy has performed various stunts in cultivating his macho image, and polls show these have won him many admirers, especially among Russian women.

In 2007, while still president, he featured in glossy magazines across the world after donning combat trousers and baring his muscular torso for photographers while on a fishing trip in the Yenisei river.

Since stepping down from the Kremlin last year to become prime minister, Putin has remained firmly in the public view, scolding oligarchs about the economic crisis and reprimanding a supermarket for over-charging customers.

Sensitive to Russia's growing environmental movement, Putin while president changed the route of a planned oil pipeline to avoid Lake Baikal, which contains one fifth of the world's unfrozen freshwater.

Scientists are studying the formation of hydrate masses deep beneath the lake and searching for economically viable ways to extract trapped gas from the crystals. One cubic metre of crystal may contain up to 160 cubic metres of methane.

Last year, a special mission set a world record for freshwater submersion by descending 1,680 metres to the lake's deepest point.