Mars appears to be dead now, but a long time ago it was an active planet. Volcanoes roared, sending floods of lava across the plains of the planet. Sometimes those flows would solidify on top, forming a hollow tube through which the lava moved. Eventually, when the volcano died away, what was left was a hollow underground corridor, called a lava tube.

Sometimes, points along that tube will collapse, forming a hole in the ground above. Called skylights, we see these on Earth near volcanoes, but they're on Mars too! What you're seeing here is just such a skylight. Under this otherwise fairly featureless plain is a lava tube, and something - perhaps a meteorite - punched a hole in it. Sand flowed down, forming the collapse pit, which is about 175 meters (600 feet) across. The hole itself is 35 meters (115 feet) across, the size of a decent back yard. You can even see the rim of the hole casting a shadow on the lava tube floor, 20 meters (60 feet) down!

Skylights on Mars are pretty cool, but they may eventually be useful. The lava tubes are big enough to support a decent size exploration base, and the ground above would protect astronauts from solar radiation. What you're looking at here might very well one day be called "home" by your descendents!

Image credit: NASA/JPL/University of Arizona

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