Film director and Lightstorm Entertainment Chairman James Cameron. (Reuters/Jo Yong-Hak)

"Titanic" director James Cameron spent his birthday underwater.

Cameron, who turned 56 on Monday, dove beneath the surface of the world's deepest lake in a submersible he used to film the wreck of the Titanic, the Foundation for the Preservation of Lake Baikal said.

Cameron boarded the Mir-1 submersible and spent a few hours in Lake Baikal's waters, the Russian-based group said.

The Mir-1, less than 8 meters (26 feet) long, is one of two submersibles Cameron used to film the Titanic in preparation for the 1997 blockbuster. Russia used the vessel in 2007 to plant a Russian flag on the sea floor near the North Pole.

On Monday, it was piloted by Anatoly Sagalevich, the director of the technical council of the preservation foundation, who invited Cameron on his first visit to Lake Baikal and gave him a hardy "hydronaut's" watch.

The sickle-shaped lake in Siberia, some 5,000 km (3,000 miles) east of Moscow, is the world's oldest and deepest lake, according to UNESCO, which lists it as a World Heritage Site. It holds one-fifth of the world's unfrozen fresh water.

Ecologists say the 25 million-year-old lake -- which reaches a depth of 1,637 meters (5,370 feet) -- harbors 1,500 plant and animal species, including a unique freshwater seal.

The two Mir craft have been used to explore the lake since 2008.

Prime Minister Vladimir Putin, who last winter signed a controversial order allowing a paper mill environmentalists say pollutes the lake to reopen, dove to the bottom of Baikal in 2008 and declared the lake clean.

Cameron, an avid deep-sea diver, has said his initial motive in proposing to make Titanic was the hope of filming the famous wreck on the floor of the North Atlantic.