Australian experts have applauded the find, saying the discovery of liquid and frozen water resources is key due to their ability to produce fuel and provide life support for astronauts during deep space missions, allowing lower cost launches and space operations.
Researchers collected more than three years of radar data from the European Space Agency's Mars Express mission.
With surface temperatures as low as minus 68C, it would not exist as a liquid under normal conditions.
Liquid water is an essential requirement of life as we know it.
"The liquid water is not a lake that you would want to swim in, locked away 1.5km beneath the surface of the Martian South Pole, the water would be a brine mixed with perchlorate salts".
"Ground-penetrating radars use radio signals that are capable of penetrating into the ground and then get reflections from the material under the surface", Roberto Orosei, principal investigator on MARSIS and a planetary scientist at the Italian National Institute for Astrophysics, told NPR.
"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.
"Discovery of a large water supply on Mars confirms we're heading in the right direction".
If you're now asking yourself, "Wait, didn't they already find water on Mars?" then you're not wrong.
Billions of years ago, Mars is thought to have had oceans and rivers, much like Earth.
The high levels of salt in the water might make the environment tough but not impossible for life.
Last week data were released revealing that there are organic molecules on Mars. There's a good chance that similar lakes exist elsewhere, waiting to be found through similar techniques. And, say the authors, "there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location".
"That water could be sourced from asteroids, the moon, or Mars".
Still, the more measurements scientists can collect, the more confident they can be.
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 there are terrestrial organisms that can survive and thrive, in fact, in similar environments".