An ancient ecosystem that has thrived in isolation for millions of years has been discovered in a pool of dark, salty water beneath half a kilometre of ice in Antarctica.

Microorganisms in the pool evolved to live without light or oxygen after being covered by the Taylor glacier on the East Antarctic ice sheet up to two million years ago. Scientists estimate the pool's temperature to be around -10C, but the water does not freeze because it contains so much salt – around four times as much as seawater.

The discovery of simple organisms in the unmapped reservoir provides further evidence of the extreme conditions that life might be able to endure on other planets.

"This briny pond is a unique time capsule from a period in Earth's history," said Jill Mikucki, who led the research at Dartmouth College in New Hampshire, US. "I don't know of any other environment quite like this on Earth."

Scientists made the discovery while analysing water samples from Blood Falls, a curious blood-red stain on the face of the Taylor glacier. Explorers in the early 20th century thought the stain was caused by red algae, but subsequent investigations have revealed that the colour comes from rust in the water.

While the glacier is made of frozen fresh water, water samples from Blood Falls are exceptionally salty and rich in iron and sulphur, but contain no oxygen. Water from the subterranean pool, which is thought to be around 5km wide, seems to be drawn up into the glacier before seeping from a tiny outlet in its face four kilometres away.

Because water flows only erratically out of the glacier, it took the researchers years to get enough samples to analyse. In the water eventually collected by the team, Mikucki found 17 different types of marine microbe, including a bacterium called Thiomicrospira arctica, though she suspects around 30 types might live in the pool. The study is reported in the journal Science.

"When I started running the chemical analysis on it, there was no oxygen. That was when this got really interesting. it was a real eureka moment," said Mikucki.

The scientists believe the pool's microbes eke out a living by "breathing" iron leached from the bedrock beneath the glacier, using sulphur as a catalyst. With no sunlight to power photosynthesis, the microbes are thought to feed on organic matter that was trapped in the pool with them.

Despite their lengthy spell in isolation, Mikucki was able to culture the bacteria and extract DNA from them. Tests showed they were remarkably similar to modern marine microbes, suggesting the population living beneath the glacier was once part of a larger population living millions of years ago in the surrounding area or in an open fjord.

Studying the microbes might help to explain how life survived a period of our planet's history known as "Snowball Earth", when ice sheets encroaching from both poles met at the equator, encasing the world in ice.

"It's a bit like finding a forest that nobody has seen for 1.5 million years," said Ann Pearson, a co-author of the report at Harvard Univeristy in Cambridge, Massachusetts. "Intriguingly, the species living there are similar to contemporary organisms, and yet quite different – a result, no doubt, of having lived in such an inhospitable environment for so long."