A vast reservoir of liquid water might be hidden deep beneath Mars’ surface, within fractured igneous rocks, potentially holding enough water to cover the planet in an ocean. This conclusion comes from seismic data collected by NASA’s InSight lander, which provided insights into Mars’ interior. The water, located between 7.2 to 12.4 miles (11.5 to 20 km) below the surface, could offer conditions suitable for sustaining microbial life, either in the past or present, according to the researchers.

“At these depths, the crust is warm enough for water to remain liquid. Closer to the surface, the water would be frozen as ice,” explained Vashan Wright, a planetary scientist from the University of California, San Diego’s Scripps Institution of Oceanography, and the lead author of the study published in the *Proceedings of the National Academy of Sciences*.

Michael Manga, a planetary scientist from the University of California, Berkeley, and co-author of the study, added, “On Earth, we find microbial life deep underground where rocks are saturated with water and there is an energy source.”

InSight, which landed on Mars in 2018, studied the planet’s deep interior until the mission ended in 2022. It measured the speed of seismic waves, which varies based on the rock composition, cracks, and what fills those cracks. By combining these seismic measurements with gravity data and rock physics models—similar to those used to study Earth’s aquifers and underground resources—the researchers identified the presence of liquid water within fractured igneous rocks in Mars’ crust.

“A mid-crust with cracked rocks filled with liquid water best explains both the seismic and gravity data,” Wright noted. He estimated that if all the water in these fractures were extracted, it could fill a global ocean 0.6-1.2 miles (1-2 km) deep.

Mars, now cold and barren, was once warm and wet, with rivers, lakes, and possibly oceans on its surface over 3 billion years ago. The study suggests that much of this surface water did not escape into space but instead seeped into the crust.

“Early Mars had liquid water on its surface, and its crust could have been saturated with water from very early on,” Manga explained. “This would have occurred when the upper crust was warmer than it is today, similar to Earth’s history of groundwater infiltration.”

Water is crucial for future human missions or settlements on Mars. While Mars has water in the form of ice at its poles and subsurface, the depth of this potential liquid water reservoir makes it difficult to access. “Drilling to such depths is extremely challenging. An alternative approach could be to search for geological activity that brings this water closer to the surface, such as in the tectonically active Cerberus Fossae region,” Manga suggested, while also emphasizing the need to address environmental protection concerns on Mars.