The impact of Eyjafjallajökull
The Icelandic volcano Eyjafjallajökull spewed so much soot into the air that it shut down European airspace from 15th to 20th April, causing an apparent loss of US$ 200 million every day.
Here's a video that shows how European airspace looked during the time, put together by its makers from a whole lot of ATC data. My only reaction is, wow!
And for some spectacular pictures of the eruptions, click here, and here.