You may (should) remember the volcanic eruptions at Eyjafjallajökull (pronunciation) in Iceland. The volcano spewed lava and sent out a plume of ash several kilometres up in the atmosphere. The lightning produced in the volcanic plume created a spectacular electrical phenomenon known as a dirty thunderstorm. For fear the that the ash may damage aircraft engines, the airspace of many European countries were closed off, leaving travellers stranded. One could not simply fly into Europe.
Air travel was disrupted from April 15th to April 20th and this data visualization shows the northern European airspace during that time.