In the search for life beyond earth, few discoveries can be a greater import than finding liquid water. As far as we can tell, liquid water has served as the crux for the development of life on our planet, and it stands to reason, would be the same for life on any other. Now, a recent discovery made by the European Space Agency’s Mars Express spacecraft seems to indicate that liquid water can be found as nearby as our red neighbor Mars, hidden from the harsh surface elements beneath a mile of rock.

An underground lake, approximately 12.4 miles wide and only a few feet deep was discovered deep beneath the planet’s South Pole through the use of ground penetrating radar aboard the ESA’s fifteen-year-old satellite. While there’s little debate as to whether or not Mars was once rich in liquid water, this discovery represents the most sizable deposit of the life-giving substance as yet discovered on the Red Planet. In 2015, the discovery of what was believed to be streaks on Martial soil caused by briny groundwater seeping to the surface raised many questions about the presence of liquid water on Mars today, but subsequent analysis seemed to indicate that those streaks were actually caused by avalanches of dry, discolored sand. As such, the discovery of a liquid water lake deep beneath the surface of Mars now seems to represent the most promising evidence to date that some form of microscopic life may still exist somewhere within the planet, even if the surface has grown uninhabitable over the millennia.

The Mars Express probe has actually been orbiting Mars since December of 2003, though it wasn’t until about 18 months later that it deployed it’s impressive radar array made up of two booms each measuring around 60 feet long. Together, the booms created a whopped 120-foot antenna, allowing their Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) systems to learn more about the interior structure and composition of the planet beneath the surface that is actively being studied by surface rovers.

European Space Agency

Back in 2007, it seemed that the probe might have found signs of sub-surface liquid water also near the planet’s south pole. The radar array detected differences in the radar signals bouncing back up it commonly referred to as “echoes,” indicating a difference in composition between two layers of sediment. Those echoes seemed to be in keeping with a deposit of liquid water, but further examination revealed that they had instead discovered a pocket of frozen carbon dioxide.

“CO2 ice is very transparent and is able to let the radar pulse penetrate into the ice much better than pure water ice,” Roberto Orosei, a co-investigator of the MARSIS instrument at the University of Bologna in Italy, explained. He is now the lead author on the new study that seems to indicate real water has been found. However, the idea that there could be liquid water beneath the frozen deposits of Mar’ southern pole didn’t die there. For years, the Mars Express has searched the region as it passed overhead, compiling data and eventually leading to this recent discovery.

Because of the immense cold where the underground lake was found, it stands to reason that it must be in contact with a surface that’s heavy in natural salts. Based on its location, scientists estimate the temperature of the water to be between negative 10 and negative 20 degrees — not exactly friendly for life, but life has been found in harsher climates here on earth.

“From what I think we have learned about this subglacial lake, the most likely analogue for this environment is the subglacial lake that was recently discovered in Canada, in which the lake itself is in contact with a deposit of salt, and so it is very very salty,” Orosei said. “There are microorganisms that are capable of surviving well below zero even without being in contact with water, and there are microorganisms that can use the salt, presumably the salt in the water on Mars, for their metabolism.”

Of course, there are no guarantees that life actually exists deep in the sub-glacial lake — this discovery might be better thought of as a good place to start looking for life, rather than any sort of confirmation that it exists — but pragmatic skepticism notwithstanding, the identification of this lake could eventually lead to some significant discoveries in the future.