Before the descent, Mr Putin familiarised himself with the Mir submersible, which is on a Baikal expedition, to explore the lake from the special-purpose vessel Metropol, anchored two kilometres from the shore.

Oceanology Institute Director Robert Nigmatulin told the Prime Minister about the history of the submersions and the goals they pursue. He indicated that "there are some sites on the bottom of the lake that leak oil." Occasionally this results in oil stains on the surface of the lake, which is a "natural phenomenon and Baikal easily takes care of them," Mr Nigmatulin said.

"Industrial enterprises have not seriously harmed Baikal. The lake has the capacity to cleanse itself," Nigmatulin assured.

"What about the Baikal pulp-and-paper mill," Mr Putin asked.

"It does have a limited impact, of course, but overall the lake's ecology does not suffer as a result," Mr Nigmatulin explained.

He also told the Prime Minister about gas-hydrates, discovered during the expedition. According to Mr Nigmatulin, gas-hydrates consist of water and methane just like carbonated water, but instead of carbon dioxide they contain methane. They look like pieces of ice but they can be ignited.

"There are many of them on the lake floor. Their extraction is complicated, however. It is a strong greenhouse gas and its extraction is hazardous. Its production is a matter for future research," the researcher said.

Mr Nigmatulin offered the Prime Minister a piece of gas-hydrate to hold that was raised from the lake's bottom. "There are very few people in the world who have held gas-hydrates in their hands," Nigmatulin said.

Later, Mr Putin, wearing special gear, including a dark-blue outfit with his name on it, climbed into the submersible, and the Mir's hatch was sealed. Anatoly Sagalevich, chief researcher of the laboratory for scientific exploitation of deep-sea submersibles, Oceanology Institute, Russian Academy of Sciences, and pilot Viktor Nishcheta were aboard the submersible with the Prime Minister.

Prior to the descent, Mr Putin put a signature across his name in the submersions journal and also signed a cup, which is placed in a bag and attached to the submersible, in accordance with the expedition's "lay" tradition. "Hi Baikal," Mr Putin wrote on the cup and signed and dated it.

Having reached the deepest point of the lake's southern section (1,395 metres), the Prime Minister spoke to journalists over a hydrophone.

"The submersion is proceeding very well, there is a wonderful view from the window," Mr Putin said. "I can see Baikal's bottom, it is very clean and beautiful," he added. The Prime Minister indicated, however, that he was surprised to find that the lake's water was not transparent. "From an environmental point of view, the water of course is clean, but it looks more like a plankton soup, as I call it," Mr Putin explained.

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The descent onboard the deep-sea submersible Mir lasted over four hours. The vessel with the Prime Minister aboard, reached a depth of 1,400 metres. Mir also passed through the Obruchev fault before resurfacing.