In April and May 2010 flights into and out of European airspace were heavily disrupted. What was the cause?

In response to concerns that volcanic ash ejected during the 2010 eruptions of the volcano Eyjafjallajökull (in Iceland) would damage aircraft engines, the controlled airspace of many European countries was closed to instrument flight rules traffic, resulting in the largest air-traffic shut-down since World War II. The closures caused millions of passengers to be stranded not only in Europe, but across the world. With large parts of European airspace closed to air traffic, many more countries were affected as flights to and from Europe were cancelled.