Scientists have found a vast lake underneath the\u00a0surface of Mars, in what they are hailing as a hugely significant breakthrough in the search for alien life on the planet.\r\nThe discovery marks the first time\u00a0a stable body of water has been detected on Mars, resolving a decades long debate about whether it\u00a0has any water at all. It has long been assumed that if water was not present on the red planet, it would\u00a0almost certainly not be capable of sustaining life.\r\n\u00a0\r\nHowever, according to scientists responsible for the breakthrough, the vast lake lies\u00a0underneath Mars\u2019s southern pole and stretches 20km across.\u00a0\r\n\u201cIf these researchers are right, this is the first time we\u2019ve found evidence of a large water body on Mars,\u201d said Cassie Stuurman, a geophysicist at the University of Texas who found signs of an enormous Martian ice deposit in 2016.\r\n\u00a0\r\nMars long ago was warmer and wetter, possessing significant bodies of water, as evidenced by dry lake beds and river valleys on its surface. There had been some signs of liquid water currently on Mars, including disputed evidence of water activity on Martian slopes, but not stable bodies of water.\r\nThe discovery was found by scientists using the\u00a0Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) instrument on the Mars Express spacecraft. That instruments sends out radar pulses that penetrate the surface and ice caps on Mars, and measures the radio waves when they come back to the spacecraft.\r\nScientists led by\u00a0Roberto Orosei\u00a0examined that data between\u00a0May 2012 and December 2015 and found that there was a very sharp change in the radar signals, when looking at the area, which was found 1.5km beneath the surface.\r\nThe researchers spent two years ensuring that the data wasn\u2019t the result of some other effect \u2013 eventually ruling out every possible explanation apart from liquid water of some kind.\r\n\u00a0\r\n\u201cIt\u2019s tempting to think that this is the first candidate place where life could persist\u00a0on Mars,\u201d said Orosei.\r\nThe data is similar to that seen when radars look at\u00a0the\u00a0lakes of liquid water found beneath the Antarctic and Greenland ice sheets on Earth. Scientists have suggested that microbial life could live in those extreme conditions \u2013 as it might on Mars.\r\n\u201cThis kind of environment is not exactly your ideal vacation, or a place where fish would swim,\u201d Orosei added. \u201cBut there are terrestrial organisms that can survive and thrive, in fact, in similar environments. There are microorganisms on Earth that are capable of surviving even in ice.\u201d\r\n\u00a0\r\nScott Hubbard, a professor of astronautics at Stanford University who served as Nasa\u2019s first Mars program director in 2000, called it \u201ctremendously exciting\u201d.\u00a0\r\n\u201cOur mantra back then was \u2018follow the water\u2019.\u00a0That was the one phrase that captured everything,\u201d Hubbard said. \u201cSo this discovery, if it stands, is just thrilling because it\u2019s the culmination of that philosophy.\u201d\u00a0\r\nIt is even colder on Mars than it is in the Antarctic and Greenland, making the discovery of liquid water even more surprising.\r\n\u00a0\r\nThe water itself is likely to be a briny sludge. The\u00a0salt that makes that brine is also probably the reason there is liquid water at all, since it helps melt the otherwise icy water in the same way grit is used to remove snow from streets.\r\n\u201cThis is a stunning result that suggests water on Mars is not a temporary trickle like previous discoveries but a persistent body of water that provides the conditions for life for extended periods of time,\u201d said\u00a0Alan Duffy, an astronomer from Swinburne University and the lead scientist of Australia\u2019s Science Channel.\r\nThere is nothing particularly special about the area where the water was found \u2013 it is simply the region on Mars where the radar that made the detection is most sensitive. That means that there could be similar deposits under the ground all the way across Mars.\r\nIt remains to be seen if more subsurface reservoirs of water will be found or whether the newly discovered one is some sort of quirk, Orosei said.\r\nIf others are detected and a network of subglacial lakes exists like on Earth, he said, that could indicate liquid water has persisted for millions of years or even dating back to 3.5 billion years ago when Mars was a more hospitable planet.