During this process, which Shepherd says has only happened on a handful of occasions, mechanics enter an airlock at the front of the TBM. The machine is slowly filled with compressed air until the pressure equals that of the tunnel face, which happens typically around 2 bar: the same pressure a diver would experience at around 10m (33ft) below the surface.After a short period of acclimatisation, they can leave the TBM through a small hatch in the cutter face and return to their work. Decompression takes much longer and – as with scuba divers – varies according to the amount of time the mechanics have spent at the tunnel face.

The route that the TBM takes is exceptionally carefully pre-planned. The ground it passes through has been meticulously probed and mapped and an advanced laser guiding system is used to help ensure the machine stays on track. Talking to Shepherd, it’s clear that there’s not even the faintest glimmer of concern that the ends won’t meet up.