Above: Two maps display the thickness of suspected ice deposits beneath the surface of Mars. The top panel assumes the deposits are buried by a 300-meter layer of sediments; the bottom panel assumes a burial depth of 600 meters. Credit: Planetary Science Institute/Smithsonian Institution

The European Space Agency’s Mars Express probe has discovered enough water beneath the Martian equator to submerge the entire planet in a shallow ocean if melted. It’s not the first time signs of water ice have been found in this region, but this latest discovery is by far the largest deposit detected near the equator. The water ice, which is heavily contaminated by dust, stretches 2.3 miles underground and is capped by a thick shell of ash and dry dust. The Mars Express probe has been operational for about two decades—its main mission to discover water on the Red Planet that could support human and robotic explorers.

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