Delta has been flying empty planes (so-called “Ghost Planes”) to Heathrow to be sprayed with anti-malaria disinfectant to meet Australian regulations for its flights to Sydney. The Australian government requires planes to be sprayed with insecticide to fight malaria, but the US government does not permit the treatment on US soil. The closest airport where it can be done is Heathrow, so Delta has been flying empty planes to the UK so they can be disinfected and sent back to Australia.
In Europe, this has caused a bit of an environmental uproar, as people complain about the environmental impact of flying an empty plane for 8,000 miles round trip. Delta says it will be switching its disinfecting to a Chinese airport at the end of the month (why doesn’t that make me feel any better?)