Drone footage from inside the Crossrail works has revealed the true extent of the undertaking.
The BBC was given special permission to fly a camera-laden craft below the streets of London and have released a 90 second excerpt online.
The drone descends into a cavernous hole and then makes its way around a labyrinth
The footage finds people – dwarfed by the concrete structure – busy at work in their hi-vis jackets in an eerie, half-lit subterranean world.
Meanwhile, drones and model aircraft have been banned from Greenwich Park.
Signs have been erected across the capital's royal parks stating: "The flying of drones or model aircraft in the park is prohibited. By order of the Secretary of State."
A spokesperson for The Royal Parks said: "We recognise their growing popularity, but they can have a very negative impact on the wildlife in the parks and interfere with the safety and comfort of park visitors."