To lift these hefty machines - they're 650 tonnes each - Transport for London (TfL) has wheeled in a huge 750-tonne crane.
The two machines, named Helen and Amy by local school children in honour of the first British astronaut Helen Sharman and aviation pioneer Amy Johnson, will create two 3.2km underground tunnels to extend the Charing Cross branch of the Northern Line.
The new, improved route will go from Kennington to Battersea Power Station via Nine Elms station. Tunnelling will take six months to complete, while the extension should be completed in 2020.
The mayor has said the extension will be "a real boost to south London, with the improved transport link helping to provide thousands of homes and jobs for Londoners".
This is the first major Tube extension since the Jubilee Line in the late 1990s.
In pictures: The two boring beasts ahead of starting tunnelling
As for the next step: well, now Amy and Helen have to be assembled within two 77m long launch tunnels, before starting their journeys towards Kennington next week. When complete, the two machines will be 100m each in length.
Once they've been constructed, a conveyor belt system will be built to take the spoil from the tunnels up to barges on the River Thames. More than 300,000 tonnes of earth will be excavated by Helen and Amy.
£1bn in funding for the extension will be provided by the private sector through a package agreed by Battersea Power Station, Wandsworth and Lambeth councils, the mayor and government.
Mark Wild, managing director of London underground, said the extension "will bring Battersea and surrounding areas to within 15 minutes of the West End and City".
Here's how the extension will look:
Timeline of the Northern line extension:
- 2017 - Main tunnelling takes place
- 2019 - Station fit out
- 2020 - Testing and commissioning; extension in operation