Astronomers have found compelling evidence that there is a huge reservoir of liquid water buried a mile under the ice near the south pole on Mars.

Radar measurements taken from the European Space Agency’s Mars Express orbiter spotted the 12-mile-wide stretch of water at the base of a thick slab of polar ice in a region known as Planum Australe.

It is the first time that researchers have identified a stable body of liquid water on the red planet. The finding raises the likelihood that any microbial life that arose on Mars may continue to eke out a rather bleak existence deep beneath the surface.

Roberto Orosei , an astronomer and physicist at the National Institute of Astrophysics in Bologna, Italy said:  "We discovered water on Mars". Any other explanation for the bright reflections the scientists saw in their radar observations was “untenable”, he added. Details of the finding are reported in the journal, Science.

For the latest discovery, researchers in Italy analysed three years of data gathered by an instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or Marsis, onboard the Mars Express probe. The measurements, collected between 2012 and 2015, show that radar waves penetrated the clear ice at the Martian south pole but reflected strongly off a body of water that lay beneath.

The scientists, however, cannot tell how deep the water is. The radar pulses from the Marsis instrument bounce back off the water but don’t reach the bottom of the reservoir. The best the scientists can say is that the water is at least a metre or two deep.

What they do know is that the water is extremely cold. At the bottom of the ice at the Martian south pole, the temperature is estimated to be about -68 C. Though far below zero, the water is thought to remain liquid because it is under pressure and rich in salts, in particular magnesium, calcium, and sodium compounds known as perchlorates, which are readily found on the surface of Mars.