Observations from Europe's Mars Express mission have revealed that an ancient system of interconnected lakes was once present beneath the surface of Mars. Five of these ancient lakes may contain minerals crucial to life.

Models had predicted that substantial groundwater must have existed on Mars but this is the first time that the global network has been observed. Researchers have found evidence of ground upwelling in basins found in the planet's northern hemisphere. The new study is published in The Journal of Geophysical Research: Planets.

“Early Mars was a watery world, but as the planet’s climate changed this water retreated below the surface to form pools and ‘groundwater’," lead author Francesco Salese, from Utrecht University in the Netherlands, said in a statement. "We traced this water in our study, as its scale and role is a matter of debate, and we found the first geological evidence of a planet-wide groundwater system on Mars.” 

The team used observations of 24 deep craters and found features that could only have formed in the presence of water. The team saw channels and valleys carved into the walls and floor of the craters, deltas and fan-shaped sediment deposits, as well as ridged terraces. The water level present in these underground lakes is consistent with the shoreline expected for the vast ocean that once existed on Mars.

“We think that this ocean may have connected to a system of underground lakes that spread across the entire planet,” added co-author Gian Gabriele Ori, director of D’Annunzio University's International Research School of Planetary Sciences. “These lakes would have existed around 3.5 billion years ago, so may have been contemporaries of a Martian ocean.”

In five out of the 24 craters, the team detected traces of clays, carbonates, and silicates. These minerals have been linked to the emergence of life on Earth, so perhaps they had the same effect on Mars. While their presence doesn't imply that life actually formed there, the lake system remains an important location. Life, if it existed, might have moved underground and evidence of its existence might still be present below the surface.

Recently, Mars Express discovered the presence of an underground lake near the Red Planet’s south pole. And it might not be the only one.