Because of a large plume of dust being blown to the UK by a volcanic eruption in Iceland all Aircraft in the UK and beyond have been grounded, no airplanes are flying at all, they will not be flying till tommorow at the earliest , and then maybe not. Apparently the dust can cause engines to stall, which has happened before.
Heathrow is the busiest international airport in the world, and second busiest overall, the dislocation of passengers all over the world is massive.
http://news.bbc.co.uk/1/hi/uk/default.stm
David