Russian Prime Minister Vladimir Putin plunged into the depths of Lake Baikal aboard a mini-submersible on Saturday in a mission that adds a new dimension to his macho image.

Putin, a judo black belt who has flown in a fighter aircraft and shot a Siberian tiger in the wild, descended 1,400 metres (4,600 ft) below the surface of the world's deepest lake to inspect potentially valuable gas crystals.

"I haven't seen anything like that in my whole life. This is a very special feeling," Putin told reporters on emerging from the deep-sea craft looking pale and a bit dizzy after spending more than 4.5 hours underwater.

Asked if he planned to travel in space after his supersonic flight and deep-sea dive, Putin, dressed in blue overalls, said, "No, there is enough work here on earth." Hidden on Lake Baikal's largely unexplored floor are large deposits of clathrate hydrate, crystals packed with one of Russia's most lucrative exports: natural gas.

Scientists estimate Baikal hydrates contain over 1 trillion cubic metres of natural gas, an amount comparable with the world's largest discovered gas fields. Mineral extraction is banned in Lake Baikal which is a nature reserve.

Before locking down the hatches on the Mir-2 submersible, Putin was shown specimens of crystals bubbling 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 said with a smile before entering the submersible, which then dived below the surface of the world's oldest lake.

MACHO IMAGE

The 56-year-old former KGB spy has cultivated his macho image, and polls show this has won him many admirers, especially among Russian women.

In 2007, while president, he featured in magazines across the world after donning combat trousers stripped to the waist while on a fishing trip in the Yenisei River.

Since stepping down from the Kremlin last year to become prime minister, Putin has remained in the public view.

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 since existing technologies are too expensive.

"In 50 years, when other energy sources end, people will have to look into gas extraction from hydrates," said Oleg Khlystov, a scientist with the Lake Studies Institute, who said his team had found a cheaper method and applied for a patent.

Khlystov said Russian gas firm Gazprom was not interested in the new technology but that he had hopes for it in the future. "If Putin is interested, we will tell him (about it)," he said.

Khlystov's $8.5 million two-year mission studying the lake's depths has performed 96 dives. Last year, it set a world record for freshwater submersion by descending 1,680 metres to the lake's deepest point.