Between May 2012 and December 2015, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (or "MARSIS" for short) surveyed the region of Mars known as the Planum Australe, a 200-kilometer area on the planet's southern polar plain, which is composed of water ice, Carbon dioxide ice, and admixed dust.
One of the exciting things about this discovery is that there could be other liquid water pockets like this one - the place these scientists studied is one of the few places where there are now enough measurements to be able to detect liquid water underneath the surface.
"This subsurface anomaly on Mars has radar properties matching water, or water-rich sediments", lead study author Roberto Orosei, a principal investigator for the MARSIS experiment, said in an ESA news release.
Based on the data, the researchers concluded the water reservoir sits 1.5 kilometers beneath the surface ice at the Martian southern pole and is 20 kilometers wide.
Space agency NASA's Curiosity Rover had found that liquid water does flow intermittently on Mars and it also detected that water had existed there in the past.
To be clear, there's no sign of any actual Martian microbes swimming around, and the environment is not obviously hospitable - the water at the base of the polar cap is estimated to be minus-90 degrees F, far below the typical freezing point of water. First, scientists have discovered evidence of a buried lake; that's not the same as finding an actual lake.
"I predict future spacecraft mission to Mars and engineers are going to be working very hard to design methods to sample this remote water source from this newly discovered subterranean Martian lake". They focused on a region below the surface of Mars' southern polar ice cap, an area suspected for the past 31 years to have an underground lake.
Do we need to go to Mars to find out what's in this lake?.
Scientists aren't even exactly sure what to call the body of water, which they detected by analyzing radar echoes gathered over three years by the orbiting Mars Express spacecraft. However, "SHARAD uses higher-frequency radio waves that cannot penetrate as deep as MARSIS can", Pettinelli says.
Almost every craft that has visited the planet has hunted for liquid water; the Mars Express craft that detected water itself had been looking for over 12 years. Taken together, they reveal a change in the structure and the composition of the material beneath the surface of Mars' south pole that so far has only one explanation: the presence of liquid water under the surface of the red planet. Researchers have also likened the discovery to Antarctica's Lake Vostok.
"This really qualifies this as a body of water".
The Italian scientists said that "bright subsurface reflections are evident within a well-defined, 20-kilometer-wide zone" that was surrounded by "much less reflective areas".
"This is now our best, albeit slim chance of discovering life elsewhere in our Solar System until the more complex missions to Europa or Enceladus, the moons of Jupiter and Saturn we also believe have subterranean water sources".
"We have long since known that the surface of Mars is inhospitable to life as we know it, so the search for life on Mars is now in the subsurface".
"They haven't seen the light of day for hundreds of thousands of years", he said. "Because of this, there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location".
This discovery of a salty subsurface lake, on an object much closer to the Sun, and much larger than a moon, is even bigger news.