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