The Moon hangs above us like a dusty old rock, devoid of life, pock-marked by craters. As static and unchanging as it seems to us now, the Moon may have looked quite different four billion years ago, in its earliest years, according to a new study that suggests the surface may have held life.

A new study published in Astrobiology, led by Dirk Schulze-Makuch, an astrobiologist at Washington State University, and Ian Crawford, a professor of planetary science and astrobiology at the University of London, says there were times in the Moon’s early days when conditions were right for simple lifeforms.

During two crucial periods of the Moon’s formation—when it was surrounded by a debris field four billion years ago and during its peak volcanic activity 3.5 billion years ago—superheated gases could have sent water vapor to the surface, priming it for life, the study says.

The researchers believe that during these outgassing periods, the Moon would have had an atmospheric pressure of 10 millibars. That’s just 1 percent of the Earth’s atmosphere, but still more than the pressure on Mars, and enough to cause water to pool on the Moon’s surface.

These conditions, combined with the theory of a magnetic field around the early Moon that would have shielded the surface from solar and cosmic radiation, may have made for a perfect environment for simple life forms to exist.

“The image of liquid water teeming with microbes on the lunar surface completely shatters the current paradigm of the Moon as a dead rock in space,” Schulze-Makuch wrote in an op-ed for Air and Space Magazine about the study. “Of course, we have to be careful not to get carried away with the idea. After all, we don’t see any of the water-modified topography on the Moon that we see on Mars.

“Then again, would we really expect to, considering that the Moon has been pounded by solar wind, cosmic radiation, and micrometeorites for several billions of years?”

Their work builds on findings from nearly a decade ago, when NASA’s lunar orbiter and satellite found evidence of water molecules on the Moon, and to further study how the ancient Moon may have looked, the researchers propose more lunar exploration. NASA claims to be planning “a series of progressive robotic missions to the lunar surface,” but for now, future missions may be up in the air after new NASA administrator James Bridenstine cancelled the only NASA lunar surface mission in April.