"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.
After ruling out other explanations, they believe that the signals could reveal a patch of liquid water, more than 12 miles across and about a mile beneath the ice.
Water is essential to life as we know it, and scientists have long sought to prove that the liquid is present on Mars.
Since everything is a competition, perhaps this exciting discovery will inspire Donald Trump to improve NASA's budget to make sure the United States wins the race to Mars.
Between May 2012 and December 2015, the Mars Express spacecraft was used to conduct a radar survey of a region called Planum Australe, located in the southern ice cap of Mars.
A team of Italian researchers on Wednesday announced they have discovered a large saltwater lake.
Mars’s southern polar ice cap
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.
Stofan says the water in the Martian underground lake is probably salty, otherwise if would freeze solid, even deep in the ground.
Liquid water hanging out beneath the planet's surface, which is too hot for it to survive, has been a suspected reality for years.
This is the south polar cap of Mars as it appeared to the Mars Orbiter Camera on the Mars Global Surveyor on April 17, 2000.
"This thrilling discovery is a highlight for planetary science and will contribute to our understanding of the evolution of Mars, the history of water on our neighbour planet and its habitability", said Dmitri Titov, ESA's Mars Express project scientist. 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'.
"This condition on Earth happens only when you observe subglacial water like in Antarctica over places like Lake Vostok, " he added.
If confirmed, this would be the most significant body of liquid water found on Mars to date. They can not see the bottom with existing equipment, but they estimate it is at least three feet deep, otherwise they would not have detected it at all. Similarly, if the Martian lake holds life forms, they're likely to be holdovers from an ancient time when Mars was more habitable. But that feature had been ascribed to a boundary between the dust-containing ice that dominates Mars' surface and a layer of pure water ice that is expected to exist below that.
This principle of following the water is key to astrobiology - the study of potential life beyond Earth. "It will open up a very interesting area of science on Mars", he says.