Traditional mining skills used to build tunnels on the Northern Line extension

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.