Researchers have found evidence of stable liquid water on Mars for the first time. A radar instrument on a spacecraft has detected a 20-km wide reservoir trapped 1.5 km beneath the ice on its South Pole, scientists said on Wednesday.

“We discovered water on Mars,” said Roberto Orosei of the National Institute of Astrophysics in Bologna, according to The Guardian. Any other explanation for the bright reflections the scientists saw in their radar observations was “untenable”, he said.

It can take years, however, to verify whether something is actually living in this body of water, Reuters reported. Researchers will need to drill through the ice to get the water sample to establish proof. The water is likely to be extremely cold. The temperature of the bottom of the ice at the Martian South Pole is around -68 degrees Celsius. Though this is not ideal habitat for life as we know it, several microorganisms probably live in that ice.

“This is the place on Mars where you have something that most resembles a habitat, a place where life could subsist,” said Orosei. “This kind of environment is not exactly your ideal vacation, or a place where fish would swim. But there are terrestrial organisms that can survive and thrive, in fact, in similar environments. There are microorganisms on Earth that are capable of surviving even in ice.”

The finding, published in Science magazine, was based on data that researchers on the European Space Agency’s Mars Express spacecraft collected between May 2012 and December 2015 with an instrument. Radar pulses emitted by the instrument can penetrate the Martian surface and ice caps.

“Quantitative analysis of the radar signals shows that this bright feature has high relative dielectric permittivity, matching that of water-bearing materials,” the researchers said. “We interpret this feature as a stable body of liquid water on Mars.”

Previous research has found possible signs of intermittent liquid water on Mars’ surface, but none of stable liquid water, according to the BBC. Scientists have earlier found that water was present on the Red Planet in the past, but significant cooling due to a thin atmosphere has frozen most of that water.