But you can bet the new results will encourage others to look. But it's a clue to how the terrain of Mars developed and to the planet's long term climate.
However, while the find is tantalizing for astrobiologists eager to find alien life, it's also a bit of a tease. For example, Nasa's Curiosity rover has explored lake beds which show there used to be water on the surface of Mars in the past.
"If these researchers are right, this is the first time we've found evidence of a large water body on Mars", said Cassie Stuurman, a geophysicist at the University of Texas who found signs of an enormous Martian ice deposit in 2016.
"This subsurface anomaly on Mars has radar properties matching water or water-rich sediments", Roberto Orosei, principal investigator of the MARSIS experiment and lead author of the paper, said in a press release.
The Mars Express spacecraft discovered the body of water beneath the southern ice cap. The study area is highlighted using a THEMIS IR image mosaic. USGS Astrogeology Science Center, Arizona State University, ESA, INAF. "This condition on Earth happens only when you observe subglacial water, like in Antarctica, over places like Lake Vostok".
The location's radar profile resembled that of subglacial lakes found beneath Earth's Antarctic and Greenland ice sheets.
Salts in the lake are believed to have kept the water - which has a temperature as low as -68C (-90F) - from freezing over. Orosei estimated the water temperature at somewhere between 14 degrees Fahrenheit (minus 10 degrees Celsius) and minus 94 degrees Fahrenheit (minus 70 degrees Celsius). The pressure also changes the melting point of water. These penetrate its surface but bounce back when they hit a boundary between different materials, such as ice and bedrock. It's less powerful in some ways, although if there were a pool of pure water, it is possible that its instruments could find it.
"It sticks out like a sore thumb in the radar data", she said.
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Artistic impression of the Mars Express spacecraft probing Mars and of Planum Australe. The process requires a detailed understanding of the radar signals that were transmitted downward toward Mars by Mars Express. They've turned to the icy regions, which contain ice caps of water and Carbon dioxide that may hide liquid water beneath.
We discovered water on Mars.
There is, and probably will continue to be, debate about the findings of this paper, simply because the implications of this discovery are enormous.
Indeed, scientists have found bacteria and other simple forms of life living in these kinds of extreme environments on our own planet, which use chemical reactions with salts and minerals to get the energy they need to live.
In May, NASA launched another spacecraft, the InSight Mars lander, that will dig under the surface after it reaches a flat plain just north of the Martian equator in November.
Finding more liquid water on Mars may mean moving beyond radar technology, which is limited to detecting large quantities of water close to the surface.
The detection of a long-standing reservoir of liquid water is thrilling evidence of an idea that first began to be debated three decades ago: that there could be water at the base of Mars' ice caps, similar to what's present on Earth.
"Nobody dares to propose that there could be any more complex life form", Orosei said.