An Eerie Drone Flight Under London's Streets

The view begins high above the London rooftops. You descend at a gentle rate, dropping down until you're at the level of the treetops and office buildings. And then further and further, towards the giant hole that has been dug yards from the busy street.

And then you descend into the hole itself, one which leads onto an enormous tunnel wider than a city street. Hard-hatted workers file through the tunnel, lit by bright industrial lights. It's a tour of one of the world's biggest engineering projects - filmed with a drone.