Image caption Weather at the site can be stormy, requiring protection for people and gear

A pioneering British expedition to sample a lake under the Antarctic ice hopes to find unknown forms of life and clues to future climate impacts.

The mission will use hot water to melt its way through ice 3km (2 miles) thick to reach Lake Ellsworth, which has been isolated from the outside world for at least 125,000 years - maybe a million.

The team hopes to be the first to sample a sub-glacial Antarctic lake.

An engineering team leaves the UK later this week along with 70 tonnes of gear.

The project, funded to the tune of £7m by the UK's Natural Environment Research Council, aims to obtain samples of the lake water itself and of sediment on the lake floor.

Image caption The heavy equipment has to be airlifted in to Antarctica, followed by a long trek over land

"Our project will look for life in Lake Ellsworth, and look for the climate record of the West Antarctic Ice Sheet," said the project's principal investigator Professor Martin Siegert from Edinburgh University.

"If we're successful, we'll make profound discoveries on both the limits to life on Earth and the history of West Antarctica," he told BBC News.

Understanding the West Antarctic Ice Sheet is crucial to forecasting future climate change impacts, as it holds enough ice to raise sea levels globally by at least 3m (10ft) and perhaps 7m (23ft).

Exploring sub-glacial lakes may also help scientists design missions to search for life on other worlds such as Jupiter's moon Europa, which is thought to feature a liquid ocean beneath a thick layer of ice.

Pushing the boundaries

Lake Ellsworth is about 10km long and 2-3km wide - about the same size as Lake Windermere, England's biggest.

Any form of life we find there, we won't have encountered before Dr David Pearce, BAS

But that is where the similarity ends.

Ellsworth lies in a valley in the bedrock of Antarctica, with 3km of ice above.

The water is kept liquid by natural geothermal heat coming from the Earth's interior.

It has been mapped by using ground-penetrating radar and seismic tests.

Among other things, those investigations revealed that the lake has a soft floor, which presumably means a thick layer of sediment.

But reaching the lake and taking samples involves a mission that pushes the boundaries of engineering skill and ingenuity.

The hot-water drill is basically a spraying device on the bottom of a hose 3.2km long.

Ninety thousand litres of pure water will be made at the Ellsworth site by heating and then filtering ice, using a boiler taken along for the purpose.

Image caption Hot water will be sprayed from the bottom of the hose, melting its way to Lake Ellsworth

It will be pumped down the 3.2km-long hose and out of the nozzles at the bottom.

With the water at a scalding 97C, it should melt a smooth, uniform hole about 36cm across down to the bottom of the ice.

A 5m-long probe will then be lowered through the hole and into the lake, carrying 24 flasks that will gather water samples at various depths.

It will also carry lights and a high-definition video camera, and filters to draw solids from the water.

Much of the equipment has been designed and built at the National Oceanography Centre (NOC) in Southampton, under the supervision of Matt Mowlem.

"This is an unknown environment - we don't know for example whether there will be dissolved gases in the water," he said.

"So the water at its pressure of 300 atmospheres will be sampled. But when we pull the probe up and the flasks hit the cold air in the borehole, the water will try to freeze; the pressure then increases to around 2,700 atmospheres, and that's greater than anything experienced in ocean engineering."

Once the probe has been hauled up, a coring device will be lowered down the borehole to take samples of the lake floor sediment.

1. A hot water drill will melt through the frozen ice sheet, which is up to 3km (2 miles) thick. After drilling, they will have an estimated 24 hours to collect samples before the borehole re-freezes 2. A probe will be lowered through the borehole to capture water samples 3. A specialised corer will then recover sediment from the floor of the lake through the same borehole Source: Subglacial Lake Ellsworth Consortium

The whole process will be a race against time.

Water on the sides of the borehole will freeze, making it progressively smaller. Professor Siegert estimates there is a window of about 24 hours to complete the dual sampling before the hole becomes too small.

First contact

No-one has yet sampled any of the estimated 387 sub-glacial lakes on Antarctica, though a Russian-led team is targeting the biggest - Lake Vostok - and a US crew is preparing to investigate Lake Whillans.

The Lake Vostok project was delayed for years over concerns that drilling might contaminate the water with organisms from the surface.

The UK team has designed its equipment and its procedures in order to avoid taking unwanted hitch-hikers down to the lake.

Image caption Russian and US teams are targeting Lakes Vostok and Whillans respectively

What might be in the water is a mystery waiting to be unravelled.

"Just about everywhere we look on the planet, we find life, from the outer reaches of the stratosphere to the deepest ocean trenches," said David Pearce from the British Antarctic Survey (BAS), who heads the search for microbiological life in Lake Ellsworth.

"Any form of life we find there, we won't have encountered before - there will probably be viruses, and we may have bacteria, archaea (other single-celled organisms) and... maybe fungi."

If the lake contains no life, said Dr Pearce, that would be interesting as well, helping to define the conditions under which life can and cannot exist.

Different worlds

How different any new organisms are from what we know already will depend to a large extent on how long Lake Ellsworth has remained isolated - in other words, for how long the West Antarctic Ice Sheet has remained intact.

Media playback is unsupported on your device Media caption Prof Martin Siegert: "Wherever there is water there is life"

This is also the key issue on the climate side of the project, which basically involves discovering how likely the ice sheet is to melt in the coming decades and centuries.

"There is some evidence from outside Antarctica that sea levels were higher at various times in the last million years - 125,000 years ago, 380,000 years ago - but we have no evidence that the water came from the West Antarctic Ice Sheet," said Professor Mike Bentley from Durham University.

"So one of the things we're looking for in our sediment core is... marine sediments that would look very different from lake sediments."

If the team can reconstruct a record of when the ice sheet melted in the past, that will give scientists a better understanding of how it is likely to behave as temperatures rise in future - and what scale of temperature rise would be needed to melt it again.

The equipment will be delivered to the Ellsworth base during the coming Antarctic summer, and stored away against the harsh winter to follow.

The main scientific party will fly out in about a year's time, unpack the equipment, and begin drilling into the unknown.

Follow Richard on Twitter