The scientists believe this bright reflection, found 1.5 kilometers deep beneath layers of ice and dust, is an interface between frozen water and a stable body of liquid water.
With surface temperatures as low as minus 68C, it would not exist as a liquid under normal conditions.
"This is just one small study area; it is an exciting prospect to think there could be more of these underground pockets of water elsewhere, yet to be discovered". Water is considered a fundamental ingredient for life.
"If life ever arose on Mars, I would say that what has been discovered today, that subsurface lake, is probably one of the best spots where we could find life", Chevrier said.
Previously, there has been some suggestions about water on Mars, like droplets of water condensing on the Phoenix lander or as the possible cause of recurring slope lineae, which are seasonal dark streaks on Martian slopes. The device sends radar signals that pierce the ice at the planet's surface, and measures how the "radio waves" spread and reflect back 'to the probe'.
Assured of their discovery, they published their findings in Science on Wednesday.
"Discovery of a large water supply on Mars confirms we're heading in the right direction".
Previous discoveries have only been around temporary trickles of water and so to have found a massive reservoir of liquid hidden underground has been hailed a stunningly fantastic result. Brine lakes on Earth can remain liquid at 8.6 degrees Fahrenheit, according to the study. However, the presence of liquid water at the base of Martian polar caps was first hypothesized in a study 31 years ago.
"We found that any other explanation for these very strong echoes was not really tenable in light of the evidence that we had available", he said.
"That water could be sourced from asteroids, the moon, or Mars".
"Caution needs to be exercised, however, as the concentration of salts needed to keep the water liquid could be fatal for any microbial life similar to Earth's", added Watson, who was not involved in the research.
Mars and Earth were closer than this year in 2003, when the two planets were just 55.7 million km apart, which was the closest in almost 60,000 years and won't happen again until 2287, NASA said. But the MARSIS team had to use a new method of getting high-resolution raw data from their instrument, then had to combine three and a half years' worth of observations-29 separate radar profiles-before they were confident in their conclusion.
"There are single-celled organisms that survive in such an environment with a metabolism that makes use of the salt", said the professor.