HS2 high-speed rail project begins underground tunnel
The largest tunnelling machine ever used on a UK rail project is officially starting work.
Florence - named after Florence Nightingale - will spend the next three-and-a-half years cutting through the chalk beneath the Chiltern hills in Buckinghamshire.
The tunnel will form part of controversial high-speed rail project HS2's first phase, which will link London with the West Midlands.
The BBC takes a look inside the operation to see how it works.
Video journalist: Samantha Everett