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The southeastern United States has been hit by a series of strange unexplained quakes - most recently, the 2011 magnitude-5.8 earthquake near Mineral, Virginia that shook the nation's capital. Researchers have been baffled, believing the areas should be relatively quiet in terms of seismic activity, as it is located in the interior of the North American Plate, far away from plate boundaries where earthquakes usually occur. Now, they believe the quakes could be caused by pieces of the Earth's mantle breaking off and sinking into the planet.