By Helen Briggs
BBC News science reporter
Antarctica's buried lakes are connected by a network of rivers moving water far beneath the surface, say UK scientists.
Hundreds of lakes lie beneath the frozen wastes of Antarctica Image: British Antarctic Survey
It was thought the sub-glacial lakes had been completely sealed for millions of years, enabling unique species to evolve in them.
Writing in the journal Nature, experts say international plans to drill into the lakes may now have to be reviewed.
Any attempts to drill into one body of water risks contaminating others.
"What this paper shows is that not only could you contaminate a lake, you could contaminate the whole drainage system," lead author Duncan Wingham, of University College London, told the BBC News website.
The sub-glacial lakes of Antarctica are regarded as "time capsules" of the period when the continent began to freeze over.
Scientists believe any life they contain might shed light on extreme environments on other worlds, such as the ice-bound ocean on Jupiter's moon Europa.
Vostok formed around 16 million years ago
The presence of the drainage system may change current thinking on the chances of finding microbial life that has evolved "independently".
"The notion that these things have been sitting in the lakes evolving for millions of years probably won't wash," said Professor Wingham.
"I think the idea that they have an isolated biological environment where things have gone their own way will have to be re-examined."
Professor Martin Siegert, of the University of Bristol, a co-author of the Nature study, said there would still be a very interesting microbiological story to uncover.
"We have always thought of sub-glacial lakes as being distinct bodies isolated from each other," he said.
"For at least some of these lakes, that won't be true but they will still be isolated from the atmosphere."
It was once thought the Antarctic continent was too cold for water to exist in liquid form beneath its frozen wastes.
But since the 1960s, satellites and aircraft with powerful radar devices have discovered more and more lakes buried kilometres beneath the thick ice sheet.
LAKE VOSTOK - ANTARCTICA
There are more than 150 sub-glacial lakes in Antarctica; Vostok is the biggest
At 14,000 sq km, it is about the extent of Lake Ontario and is up to 500m deep in places.
Overlying ice layers reveal a 400,000-year environmental record with microbes present throughout the core
Many scientists consider Vostok to be a good model for the ecosystems that might exist on Jupiter's frozen moons
More than 150 have been detected so far, but they are expected to run into thousands.
The largest underground body of water in Antarctica is known as Lake Vostok, which is 250km (155 miles) long, 40km (25 miles) wide and 400m (1,300ft) deep.
The US space agency (Nasa) and the Russian academy of sciences are planning to break through the ice to sample the water for life.
There are also proposals to explore and sample Lake Ellsworth in west Antarctica by a team involving 14 UK universities and research institutions, plus scientists from Chile, the US, Sweden, Belgium, Germany and New Zealand.
Cynan Ellis-Evans of the British Antarctic Survey in Cambridge, UK, said the idea of using a small lake away from Vostok to test methods would now have to be looked at again.
"This paper indicates that lakes may well be connected at irregular time intervals and that even an apparently isolated lake can breach and transfer water hundreds of kilometres to other distant lakes so the contamination potential in relation to small lakes is greater than we'd previously thought," he said.
But he said evidence from examining shorelines suggested larger lakes like Vostok were relatively stable.
"I am of the view that Vostok and other similar large lakes have developed in isolation and so the interest from a biological viewpoint will remain high."
The latest research was carried out by scientists at the Natural Environment Research Council (Nerc) Centre for Polar Observation and Modelling (CPOM) at UCL, the University of Bristol and University of Cambridge.
They took ultra-precise measurements of a region in East Antarctica - home to some of the oldest, thickest ice on the continent - using radars on the European Space Agency's ERS-2 satellite.
The satellite found synchronous changes in the surface height at several locations hundreds of kilometres apart.
"To find a whole section - 30km (18 miles) by 10km (6 miles) - had dropped vertically was a great surprise," Professor Wingham explained.
"We then found another similar event 300km (186 miles) away, but that bit had increased instead of decreasing.
"We were then left with the problem of explaining what was going on. Movement of water was the only mechanism conceivable."
The scientists believe that every so often there are large flows of water from one lake to another along rivers the size of the Thames.
Most of the time there is very little discharge, but if a lake over pressurises, a flood occurs that forces the water along the river to the next lake.
"You could think of these things as flushing like lavatories every now and again," said Professor Wingham.
It was once speculated that Lake Vostok, which contains enough water to supply London for 5,000 years, may have generated huge floods that reached the coast at some point in its history.
The latest research raises the prospect that the same thing could happen again.
"Currently, we don't know how full Lake Vostok is or the length of time it will take to fill," said Professor Siegert.
"It might be thousands or even tens of thousands of years. Whether such a discharge could affect the ocean circulation around Antarctica is an open question at this stage."
He said any discharge would probably take place over a period of months and would change sea level by less than a centimetre.