Mars may be hiding a vast stash of water beneath the Medusae Fossae Formation (MFF) near its equator.

This ice-rich deposit could be the largest known water reservoir in this part of Mars, with an estimated volume comparable to Earth’s Red Sea.

If melted, it has the potential to flood the entire planet in nine feet of water, scientists say.

Fresh data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) suggests the underground water reservoir extends up to 2.2 miles (3.7 km) deep—far more than previously thought.

“We’ve explored the MFF again using newer data from Mars Express’s MARSIS radar and found the deposits to be even thicker than we thought,” said Thomas Watters, lead author of the study and a scientist at the Smithsonian Institution.

“Excitingly, the radar signals match what we’d expect from layered ice, similar to Mars’ known polar ice caps.”

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