Reports of drone sightings shut down London's main airport Heathrow on Tuesday - just weeks after a similar incident caused holiday travel chaos at another major airport, Gatwick.
This video shows planes stranded on the runway at Heathrow.
The Gatwick incident affected more than 140,000 passengers over the course of three days.
But no hard evidence ever emerged that the drone flights actually happened.
Heathrow has bought systems to guard against drones, and Gatwick said it had invested several million British pounds in military-grade anti-drone defense equipment to discourage future incidents.
Drones are a safety concern because a collision with a plane can cause major damage and criminals can use them for more nefarious reasons.