For 15 million years, a vast icebound lake has been sealed deep beneath Antarctica's frozen crust, possibly hiding prehistoric or other unknown life. Now Russian scientists are on the brink of piercing through to its secrets.
They suspect its depths will reveal new life forms, show how the planet was before the ice age and how life evolved. It could even offer a glimpse at what conditions for life exist in the similar extremes of Mars and Jupiter's moon Europa.
"There's only a bit left to go," said Alexei Turkeyev, the chief of the Russian polar Vostok Station. Talking by satellite phone he said his team has been drilling for weeks in a race to reach the lake – 12,000ft (3,658m) beneath the polar ice cap – before the end of the brief Antarctic summer. It was here scientists recorded the coldest temperature ever found on earth: -89.2 C.
With the rapid onset of winter, scientists will be forced to leave today on the last flight out for this season. "It's -40C outside," Turkeyev said. "But, whatever, we're working. We're feeling good. There's only five metres left until we get to the lake so it'll all be very soon."
The Antarctic's subglacial lakes are the last great unexplored habitat on earth, and Lake Vostok is the largest of them. It is immense – 160 miles long, and 31 miles across at its widest point – a similar size to Lake Ontario, the smallest of North America's five Great Lakes.
Despite its position in Antarctica, the weight of the ice above it and the heat from the bedrocks beneath it means that scientists believe its waters may be as temperate as -3C, but liquid, and not frozen. Lake Vostok, is the largest, deepest and most isolated of Antarctica's 150 subglacial lakes. It is supersaturated with oxygen, resembling no other known environment on the planet.
Geoscientists say the ice sheet – believed to have been formed about 15 million years ago – acts like a duvet, trapping in the earth's geothermal heat. Sediment from the lake could take scientists back millions of years to tropical prehistoric times.
Some 65 million years ago, Antarctica had a climate akin to the more temperate interior of Australia, to which it was then connected. Valery Lukin of Russia's Arctic and Antarctic Research Institute in St Petersburg, which is overseeing the expedition, said: "It's like exploring an alien planet where no one has been before. We don't know what we'll find."
What all the scientists are hoping to find are whole ecosystems of hitherto unknown microbes and bacteria, trapped in a lightless world. One researcher, Brent Christner of Louisiana State University, has studied ice cores from the Lake Vostok operation and calculated that the quantity of living cells in Antarctica could exceed those in all the world's other fresh water.
And Chuck Kennicutt, an oceanographer who co-chaired a conference on these lakes last year, said then: "When it comes to understanding our planet, Antarctica is about the last frontier."
However, the discovery of what exactly are in these tantalising waters will have to wait until the next Antarctic summer, when the team can return to make the final breakthrough.
A centenary since the first expeditions to the South Pole, the discovery of Antarctica's hidden network of subglacial lakes via satellite imagery in the late 1990s has sparked a new exploratory fervour among the world's scientists.
Two other teams, including a British one, are boring down on similar, if smaller, lakes. Martin Siegert, the head of the University of Edinburgh's school of geosciences, who is leading a British expedition to a smaller polar lake, said: "It's an extreme environment but it is one that may be habitable. If it is, curiosity drives us to understand what's in it. How is it living? Is it flourishing?"
"The Russians are leading the way with a torch," said John Priscu, of Montana State University, a chief scientist with the US programme to explore another Antarctic lake. Beneath the endless white landscape, Priscu suspects creatures may lurk, far from the sunlight, around thermal vents in the depths of Lake Vostok. "I think Lake Vostok is an oasis for life under the ice sheet. It would be really wild to thoroughly sample.... But until we learn how to get into the system cleanly that's an issue," he said.
The low-lying, snow-drift buildings and radio towers of Vostok Station sit above the eponymous lake. The borehole, pumped full of kerosene and freon to keep it from freezing shut, hangs poised over the pristine lake. The explorers now face the question: "How do we go where no one has gone before without spoiling it or bringing back some unknown virus?"
"I feel very excited, but once we do it there is no going back," said Alexei Ekaikin, a scientist with the Vostok Station expedition. "Once you touch it, it will be touched forever."