A new seismic analysis has uncovered evidence that a massive reservoir of liquid water could be trapped deep beneath Mars’ surface. The findings, published on 25 April in National Science Review, indicate that a layer of porous rock between 5.4 and 8 kilometres underground might be saturated with liquid water, enough to submerge the planet beneath an ocean 520 to 780 metres deep.

This potential reservoir may help explain the long-standing mystery of what happened to Mars’ once-abundant water. Based on NASA’s InSight lander data, researchers observed seismic waves slowing at specific depths, a telltale sign that they were moving through liquid rather than solid rock.

Related: Earth’s Core Isn’t Still: Scientists Discover Solid Rock Flow in the Deep Interior

The Puzzle of Mars’ Missing Water

Mars is believed to have once been rich in liquid water, with features like river valleys and sedimentary layers pointing to a wetter past between 4.1 and 3 billion years ago. Over time, the planet lost its magnetic field, and with it, much of its atmosphere. As temperatures dropped, liquid water is thought to have escaped into space, frozen in the polar caps, or been absorbed into minerals in the crust.

Still, these known processes can’t fully account for all the water once present. This gap has puzzled planetary scientists for decades.

“This also matches estimates of the ‘missing’ water on Mars from other studies,” said Hrvoje Tkalčić, co-author and professor of geophysics at the Australian National University.

Seismic Evidence Suggests Saturated Subsurface

The latest research supports the idea that much of Mars’ water may still be underground. Seismic signals from marsquakes and asteroid impacts in 2021 and 2022 suggest a “low-velocity layer” in the Martian crust, porous rock possibly filled with liquid.

“This ‘low-velocity layer’ is most likely highly porous rock filled with liquid water, like a saturated sponge,” explained Tkalčić and co-author Weijia Sun from the Chinese Academy of Sciences.

The researchers likened the Martian layer to terrestrial aquifers, underground zones where water seeps through rock pores.

What’s Next for Mars Water Research?

While prior studies have pointed to subsurface ice or deeper reservoirs of water, this study highlights a more accessible depth for future exploration. However, confirmation will require missions equipped with advanced drills and new seismic instruments.

“Future missions with seismometers and drills are needed to confirm the presence of the water at these depths and gather more clues,” Tkalčić said.