Just a few weeks ago the Eyjafjallaj¶kull volcano in Iceland decided to vent a bit of ash in to the air causing flights across Europe to be halted. It took a week or more for airspace to get back to normal.
The video below shows a visualisation of the flight traffic around Europe over a time frame starting with next to no flights to everything up and running as normal. It’s quite fascinating to see all the traffic building up to what it is on a daily basis.
The flight data is courtesy of flightradar24.com and covers a large fraction of Europe. There are a few gaps (most noticeably France) and no coverage over the Atlantic, but the picture is still clear.