We've known for a long time that Mars has volcanoes; after all, Olympus Mons on the Martian Tharsis shield is the largest volcano in the solar system.
But there may have been a more powerful menace lurking just below the red planet's surface eons ago. A paper published last week in Nature (though the authors first reported about it in March at 44th Lunar and Planetary Science Conference) claims to have found evidence of supervolcanoes on Mars.
On Earth, a supervolcano is far larger than a regular volcano; they have huge chambers of magma that can be dozens of kilometers across (like under Yellowstone). They build slowly over hundreds of thousands of years, and erupt catastrophically, and can change the landscape and climate of nearly the entire planet.
In the northern hemisphere of Mars is a series of craters that have been thought to be from impacts. Mars is littered with impact craters, since it has little atmosphere and no running water to erode away ancient features. But the craters in question, most notably Eden Patera (85 by 55 kilometers in size, or 53 by 34 miles), are a bit odd. The entire area is actually lower than average, literally sunken, which is consistent with a supervolcano; once the magma under the surface erupts out, the ground around it collapses to fill the empty space.
Other features make it suspiciously un-impact-like. The crater isn't round, it has no rim, no ejecta (material blasted out from impact), and no central peak (which are common in large craters, like Tycho on the Moon). There are substantial flows of lava around the area, and more evidence of ancient volcanism. Also, benches, or flattened rings around the inside of the crater, are present, and these are common in volcanoes as lava lakes subside.