Photo credit: ESA/ATG medialab; Mars: ESA/DLR/FU BerlinMore

From Popular Mechanics

For decades if not centuries, we wondered whether Mars had water. Now, after a string of robotic expeditions to the Red Planet, we know the answer: It definitely used to.

All of the traces of water we’ve discovered, however, have been on the planet’s surface. Recently, though, the European Space Agency’s Mars Express satellite changed that, by finding some of the first evidence of Martian water underground.

It is tough to collect evidence of groundwater on Mars because that water is, well, underground. The rovers we’ve landed on the surface can make only

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