Coronal Mass Ejection (CME) from our Sun
A CME was behind the solar storm of 1859. It occurred in September of that year, causing the failure of telegraph systems across North America and Europe. Accompanying the storm were auroras that are normally only seen in the Arctic and Antarctic, but were visible as far south as the Caribbean.
Typically we would expect a storm of 1859's magnitude cause power blackouts and wreck satellites. But do we really know how big they can get? In 2009 NASA told us to be wary of solar storms, and warned of the dangers to America's ancient overloaded power grid. Being without power for a few months, in the developed world, is a lot more serious than most people realize.
Possibility this century = 10
Possibility in 2012 = 8
Could Ancients Predict It? = 6
TOTAL = 24