The thorny problem of how best to increase London's air capacity is some way from being solved (assuming, here, that we do need to increase capacity — that's another argument). Should we expand Heathrow, focus on other airports, or build afresh somewhere in the Thames Estuary? There are huge downsides to all of the proposals.
Recently, Heathrow set out its own schemes for a third runway. All options involve compulsory purchase of land, demolition of properties and the introduction of new approach paths over other properties. In short: less than ideal.
But here's a mad idea that could expand the airport without any new land acquisition. Build downwards.
The existing two runways sandwich a strip of land about 2.5 miles long and half a mile wide. It's used for taxiing, terminals and other infrastructure. Does this all need to be above ground?
Vast hangars and tunnels could be built beneath this space, using the local expertise gained on Crossrail, super-sewer digs and other large subterranean projects. The reclaimed surface land could be used to build two new runways, potentially doubling capacity. The planes would remain underground, waiting their turn for take-off, before emerging onto the tarmac for flight. Think of it as a super-sized aviation version of Canary Wharf tube station, or a civilian scale-up of an aircraft carrier's hangar bay.
All of this, of course, would come at significant cost. But so would building a new airport in the Estuary.
Maybe we could go even further and also build the runways underground, with brief apertures at either end to allow the planes to land and take-off. It sounds madcap (and it is), but it's not unprecedented. Putting everything underground would also free up huge tracts of land on the surface, which might be sold off to part-fund the project.
Disclaimer: The above ideas are intended as a conversation piece, written from a position of total ignorance about the aviation industry, rather than any kind of serious proposal!
See also: Our April Fool's piece on piggy-back planes into Heathrow.