Deep beneath the many hundreds of metres of ice that envelope the continent of Antarctica lies a frozen landscape which has been cut off from the rest of the living biosphere for tens of millions of years. In 2007 - International Polar Year - scientists hope to shed some light on the mysteries of this lost world.
One of the great unresolved questions about the glacial continent of the South Pole is whether there is any life in a huge Antarctic lake covering an area twice the size of Yorkshire. Lake Vostok, which lies buried under an ice sheet four kilometres deep, was discovered beneath a Russian research station more than 30 years ago but hints of its existence were only confirmed by satellite and seismic measurements in the early 1990s. It was only then that its immense size became apparent.
Bigger than Lake Ontario, Lake Vostok is estimated to be up to 500 metres deep and is so voluminous that it could provide London with its entire water supply for 5,000 years. But the most interesting aspect of Vostok is that it appears to have been hermetically sealed off from the rest of the environment for at least 15 million years - the point when it became entombed in ice.
Lake Vostok is now known to be just one of about 150 subglacial lakes locked beneath the giant ice sheets of Antarctica. But it is the largest of the known freshwater lakes and the one that has generated the greatest interest in terms of the search for undiscovered Antarctic life.
The crushing pressure from the thick East Antarctic ice sheet - the biggest in the world - keeps Vostok from freezing up. Only the top few feet of the lake directly beneath the base of the giant ice sheet are thought to be frozen, a layer known as accretion ice.
No daylight penetrates to the lake, so any life in it must rely on a source of chemical energy to keep it alive in the darkness. Scientists believe that geothermal vents emerging from the bed of the lake may provide the "food" to sustain a vibrant menagerie of microbial life forms. But, without directly sampling the lake, this is mere speculation.
As it happens, the lake lies directly underneath the Russian Antarctic research station of Vostok. The base was established in the Cold War, long before Soviet scientists became aware of the lake's existence. The Vostok station also happens to be the coldest place on Earth, with winter temperatures plummeting below minus 80C.
Russian scientists began drilling through the ice many years ago using a dirty method that involves filling the drill hole with kerosene to stop it from freezing up (kerosene has a freezing point much lower than water). The scientists came to within a few metres of the liquid water of the lake when, eight years ago, they halted operations after they hit accretion ice.
Understandably, they and other Antarctic scientists were worried that the drilling would contaminate the lake irreversibly with kerosene and any microbes on the unsterilised drilling equipment. An analysis of accretion ice samples suggested tantalisingly that it contained the signs of microbial life. But the findings could not be confirmed.
Last year, after an eight-year hiatus, a joint team from Russia and France began drilling once more with a different system that uses a sterile "buffer fluid". This fluid is heavier than kerosene and was designed to sink to the bottom of the drill hole to guard against contamination of the lake. Next year, the scientists hope to break through the last few feet of ice and bring frozen samples to the surface to test for microbial life. It is an ambitious plan but not everyone is happy with it. "The first rule about working in the Antarctic is that if anything can go wrong in Antarctica, it will," says Cynan Ellis-Evans, a scientist with the British Antarctic Survey in Cambridge. "They say they're not going to contaminate the lake, but that's assuming that everything works."
There is, however, another reason why the plan may not be such a good idea. An essential assumption about Lake Vostok is that it has been isolated from the rest of the biosphere for millions of years. If so, any life in the lake would have evolved separately from the rest of life for a longer period of isolation than any other known organism.
But British scientists discovered last year that some of the underground lakes of Antarctica are connected by a network of rivers that may be in contact with the rest of the biosphere. Apart from questioning the central assumption about life in the lakes being evolutionarily isolated, it also raises the problem of whether the entire water network may be contaminated by a mishap in just one lake.
A team led by Duncan Wingham of University College London found that massive rivers the size of the Thames can flow from one lake to another for hundreds of miles beneath the ice. "Previously, it was thought water moves underneath the ice by very slow seepage," he says. "But this new data shows that, every so often, the lakes beneath the ice pop off like Champagne corks, releasing floods that travel very long distances."
The British team, which included researchers from the Natural Environment Research Council's Centre for Polar Observation and Modelling, discovered the underground rivers of Antarctica by observing that as one region of ice at the surface sank, another region some distance away rose. The changes were evidently caused by the rather sudden and dramatic movement of huge volumes of water beneath the ice sheet.
"The lakes are like a set of beads on a string, where the lakes are the beads connected by a string or river of water," Professor Wingham explains. "For the most part, there is very little flow along the string. Then, one of the lakes over-pressurises and a flood occurs that fills the next bead down the string. Once it starts to flow, it melts the ice and there is a runaway effect. Whether that could start an immediate chain reaction down the string - and hence to the coast - or whether that bead would go off sometime later is a vital question to which we don't yet know the answer."
The new discovery raises fresh questions about the wisdom of drilling through to Lake Vostok with equipment that cannot be guaranteed to be sterile. As Professor Wingham says: "New microbes could be introduced. Our data shows that any contamination will not be limited to one lake, but will, over time, extend down the length of the network of rivers. We had thought of these lakes as isolated biological laboratories.
"Now we are going to have to think again."
A continent in numbers
* Antarctica is 58 times the size of the UK and is the highest, coldest and windiest continent on Earth.
* Its icecap contains almost 70 per cent of the world's fresh water and 90 per cent of the world's ice.
* Despite so much water, its low snowfall makes Antarctica technically a desert.
* Less than 1 per cent of the land of Antarctica is free of ice and snow.
* Ice data: mean thickness, 1,829 metres; total volume of ice, 25.4 square kilometres.
* Highest mountain: Mount Vinson (4,892 metres) in the Ellsworth range.
* Length of coastline: 45,317 kilometres.