Last month, London's Gatwick airport (the second busiest airport in the UK) was closed for several days after drones were sighted in its airspace.
The Gatwick drones are still a mystery (it's even possible that there were no drones, or that the drones people saw were police drones flown to spot drones).
But as of now, London Heathrow, the nation's busiest airport, is shut due to a drone sighting.
Using drones to shut down major international airports is a hell of an asymmetrical attack — but even more efficient is phoning in a fake drone sighting and waiting for everyone else to hallucinate more drones in the aftermath.
A Heathrow spokeswoman said the airport was working with police to "prevent any threat to operational safety".
She said: "As a precautionary measure, we have stopped departures while we investigate. We apologise to passengers for any inconvenience this may cause."