Russian scientists are on the brink of piercing through to the secrets of an icebound lake which has been sealed deep beneath Antarctica's frozen crust for 15 million years.
Working in summer temperatures of -40°C, the team from the Russian Arctic and Antarctic Research Institute (AARI) had hoped to drill through to the lake this Antarctic summer.
Lake Vostok, is the largest, deepest and most isolated of Antarctica's 150 subglacial lakes. It is thought to be supersaturated with oxygen, resembling no other known environment on Earth.
They say the lake, which has stayed liquid 3750 metres beneath the polar ice cap due to geothermal heat, may be hiding prehistoric or unknown life.
"There's only a bit left to go," says Alexei Turkeyev, chief of the Russian polar Vostok Station.
His team has drilled for weeks, in a race to reach the lake before the end of the brief Antarctic summer.
The rapid onset of winter has forced the team to leave without reaching their goal. Turkeyev says they had just 5 metres to go, but they had to leave the continent by 6 February.
The Russian station above the lake holds the record as the coldest place on Earth, recording -89.2°C.
The borehole, pumped full of kerosene and freon to keep it from freezing shut, hangs poised over the pristine lake.
Scientists suspect the lake's depths will reveal new life forms, and perhaps show how the planet was before the ice cap formed and how life has evolved since. It could offer a glimpse at what conditions for life exist in the similar extremes of Mars and Jupiter's moon Europa.
Like exploring an alien planet
"It's like exploring an alien planet where no one has been before. We don't know what we'll find," says Valery Lukin of AARI.
Sediment from the lake could take scientists back millions of years to tropical prehistoric times, Lukin says.
The discovery of Antarctica's hidden network of subglacial lakes via satellite imagery in the late 1990s has sparked a new exploratory fervour among scientists the world over.
US and British explorers are on the trail of Russia's scientists with missions to probe other buried lakes, some of the last unexplored areas of the planet.
Martin Siegert, head of the University of Edinburgh's School of Geosciences, is leading a British expedition to a smaller polar lake.
"It's an extreme environment but it is one that may be habitable," he says.
"If it is, curiosity drives us to understand what's in it. How is it living? Is it flourishing?"