By Katie Oakes, ITV London
Miners are tunnelling part of the Northern Line extension by hand in an operation that's too dangerous and delicate for big machines.
The £1.2bn extension linking Kennington to Battersea is due to be completed in 2020.
Part of the work involves excavating around the 100-year-old casing of the existing Northern line while trains are still running.
They tunnel 24 hours a day, seven days a week but progress is slow. It takes a week so progress 2.4m, compared to giant boring machines which can tunnel 30m a day.
Many of the miners working on the new tunnels are using skills passed down through the generations from the men who built the Underground.