MOSCOW/IRKUTSK, August 4 (RIA Novosti) - Scientists using two Russian mini-submarines to explore Siberia's Lake Baikal set off on Tuesday for the north of the lake for the last stage of their summer research, an expedition participant said.

"The expedition left for the Nizhneangarsk area where it will work till September," he said.

After conducting celebrity tours on Sunday and Monday for Prime Minister Vladimir Putin and rock star Andrei Makarevich, the Mir submersibles were checked and their batteries charged before setting off.

The expedition is expected to arrive at the new research site on Thursday having traveled about 500 kilometers (310 miles), a spokesman for the expedition said. Geothermal processes and the lake bottom will be examined during the dives in Baikal's north.

The first part of the expedition was conducted in the summer of 2008. This summer, the Mir-1 and Mir-2 mini-submarines have searched for new flora and fauna species, as well as diving to the lake's deepest point near Olkhon Island.

On Sunday, Putin spent about four hours in the Mir-1 submersible, diving to 1,395 meters in one of the deepest parts of the lake.

In communication with journalists on the surface, the prime minister expressed surprise at the poor visibility, describing the water as "plankton soup," but he declared himself happy with the lake's ecological condition.

"The water is of course pure from an ecological point of view," Putin said. "The bottom of Baikal is visible."

The submersibles will continue working in Baikal for one more month before returning to Kaliningrad, where their home ship Akademik Mstislav Keldish is located.