Apparently, the discovery was made using Marsis (Mars Advanced Radar for Subsurface and Ionosphere Sounding), a radar instrument that is used by scientist to study beneath a planet's surface. This means that scientists can't specify whether it's an underground pool, an aquifer-like body, or just a layer of sludge. Water is considered a fundamental ingredient for life.
Finding that much water could be a great signpost in our search for life on Mars, but the results should be taken with a grain of salt.
Mars long ago was warmer and wetter, possessing significant bodies of water, as evidenced by dry lake beds and river valleys on its surface.
"On Earth, it's nearly a given that if a ground-penetrating radar spots stronger reflections from the sub-surface than from the surface of the polar ice, then you are seeing liquid water".
"Since United Launch Alliance put a price on delivering water in space in 2016, researchers, agencies and companies have focused on water, for support of life and chemical processes, and for conversion to hydrogen and oxygen for rocket fuel", he said.
"This is the place on Mars where you have something that most resembles a habitat, a place where life could subsist", said planetary scientist Roberto Orosei of Istituto Nazionale di Astrofisica in Italy, who led the research published in the journal Science.
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 incredible result.
The temperature is likely below the freezing point of pure water, but the lake can remain liquid due to the presence of magnesium, calcium and sodium. If it is a lake, it's probably several times saltier than Earth's oceans-more of a brine. There's a good chance that similar lakes exist elsewhere, waiting to be found through similar techniques. But there hasn't been evidence of stable bodies of water until now, the researchers said.
"That water could be sourced from asteroids, the moon, or Mars".
Still, she said, there are microbes on Earth that have been able to adapt to environments like that.
"We don't see the same reflector with SHARAD, not even when we recently summed together [thousands] of observations to create CATSCAN-like 3-D views of both polar caps", Nathaniel Putzig, Mars Reconnaissance Orbiter SHARAD deputy team leader and senior scientist at the Planetary Science Institute, said in an email.
"There are single-celled organisms that survive in such an environment with a metabolism that makes use of the salt", said the professor.