A "major milestone" was reached this week in the Northern Line Extension project when Amy and Helen, two giant tunnel boring machines, were lowered 20 metres below ground.

A 750-tonne crane was needed to lift the two machines, which will create 3.2km underground tunnels extending the Charing Cross branch of the line from Kennington to Battersea Power Station.

Tunnelling will begin in March and take six months to complete.

Once lowered, Helen and Amy can be fully assembled within two 77 metre long launch tunnels and their final length will be a whopping 100 metres.

The machines were named by school children after the first British astronaut, Helen Sharman, and British aviation pioneer, Amy Johnson, the first female pilot to fly alone from Britain to Australia.

The earth- more than 300,000 tonnes- the machines dig out will be taken up to barges on the River Thames and be transported to Goshems Farm in East Tilbury, Essex, where it will be used to create arable farmland.