A team of scientists have discovered a 12.5-mile-wide lake beneath the surface of Mars’ southern polar ice cap.

A “stable body of liquid water” has been found to occupy a “well-defined, 20-kilometer-wide zone” beneath the surface of the Red Planet. The discovery was made by a team of scientists led by Professor Roberto Orosei using the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS).

The team used MARSIS to survey Mars’ “Planum Australe” region from May 2012 to December 2015 and has finally announced “radar evidence of subglacial liquid water on Mars” in a study published to Science.

Unlike many Mars discoveries, this particular revelation is all but definitive. Further, it is a real lake — as opposed to simple runoff from the massive ice above it. “This really qualifies this as a body of water. A lake, not some kind of meltwater filling some space between rock and ice, as happens in certain glaciers on Earth,” Orosei told the BBC.

This discovery represents the best lead so far on potential Martian life. While Mars’ surface continues to be classified as “inhospitable to life,” this pocket of unfrozen liquid could harbor alien microbial life. For that matter, Orosei said, “this is just one small study area; it is an exciting prospect to think there could be more of these underground pockets of water elsewhere, yet to be discovered.”

Dmitri Titov, Mars Express project scientist for the European Space Agency, enthused about the “much-awaited result” of Orosei’s work. “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 neighbor planet and its habitability,” he said.