The probes have already set world records
Russian scientists say they have reached the bottom of the world's deepest body of fresh water - Lake Baikal in Siberia.
However, they did not break the record for the deepest freshwater dive, as was originally reported.
The Mir-1 submarine descended 1,580m (5,184ft), organisers said, and not a record 1,680m as was earlier claimed.
The mission is part of a research project at Lake Baikal, which contains about 20% of the world's fresh water.
The lake was declared a Unesco World Heritage site in 1996.
Russia's Interfax and Itar-Tass news agencies cited expedition organisers as saying that the Mir I and Mir II mini-submarines had touched the bottom of the lake.
"This is a world record for a submarine diving in fresh water," Interfax quoted an organiser as saying.
However, it was later confirmed that the dive did not go as deep as first thought.
"There was no record... We'll try again," expedition leader Artur Chilingarov, a Russian parliamentarian and celebrated Arctic explorer, said after the dive.
One of the submarines is lowered into the water, ready for the mission
The mission team plan to make up to 60 dives in total.
The current record of 1,637m was set in Lake Baikal in the 1990s.
The Mir capsules are already in the record books for their undersea expeditions - descending to depths greater than 4,000m.
Before the expedition set off, Mr Chilingarov described it as a complex one.
"There are technological problems, fickle weather conditions. Fresh water dictates its own special conditions," he said.
The two 18-tonne mini-submarines were designed to operate in seawater - but have shed hundreds of kilos to make them buoyant enough in less dense fresh water.
Mr Chilingarov also led a team of scientists to the North Pole in August last year - where they controversially staked Russia's claim by planting a flag on the seabed.
The BBC's James Rodgers, at Lake Baikal, says the latest expedition is another sign of the Kremlin's desire to show the world the kind of feat a newly confident Russia is capable of.
Environmentalists had expressed concern that Russia intended to exploit the lake's mineral wealth, but expedition leaders insisted the mission was for research and conservation purposes only.
Mr Chilingarov said his team would put together "a package of practical measures and recommendations" to promote conservation of the lake.
Lake Baikal, formed 25 million years ago, is home to hundreds of species of flora and fauna.
One of the most unusual animals unique to the lake is the Baikal seal - one of the few seals to spend its life in fresh water.