For this water to remain in a liquid state, the team believes that there are various salts saturating the liquid, keeping it from freezing over.
"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.
When most people talk about water on Mars, they're usually talking about ancient water or frozen water.Now we know there's more to the story.
Between 2012 and December 2015 the Planum Australe region was mapped by the Mars Advance Radar for Subsurface and Ionosphere Sounding (Marsis) instrument carried on the orbiter. 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'. Until now, scientists had only confirmed significant amounts of water in the form of ice, primarily located at the poles.
"This could be, perhaps, the first habitat we find on Mars", said Orosei, who led the study published in the journal Science, according to The Washington Post.
"Quantitative analysis of the radar signals shows that this bright feature has high relative dielectric permittivity (electrical polarisation) matching that of water-bearing materials".
There have been previous signs of liquid and water activity on Martian slopes, but never before a "stable body" - which strengthens the notion of alien life inhabiting the planet. The radar cross section has been tilted 90°.
"This water would be extremely cold, right at the point where it's about to freeze".More news: Pres. Trump, European Union leaders announce path forward on trade
"The underground water might exist as a lake trapped beneath rock layers or mixed in with Martian soil to create a salty sludge, but either way at 20km across there is a lot of it".
Either way, the discovery greatly increases the chances of extraterrestrial life existing on Mars.
A massive underground lake has been detected for the first time on Mars, raising hopes that more water - and maybe even life - exists there, global astronomers said Wednesday.
Researchers said the lake looked similar to the ones found beneath the Antartic and Greenland.
The discovery is the latest of many breakthroughs by Nasa's Mars missions. But there hasn't been evidence of stable bodies of water until now, the researchers said.
"There are likely similar water deposits below the ground all across Mars".
The radar data showed that the lake contained a large amount of salt, Prof Orosei added.