The North Downs Tunnel was constructed as one element of the first phase of the Channel Tunnel Rail Link (CTRL), which was subsequently rebranded as High Speed 1. Its north-west portal, the nearest to London, is positioned at the head of the Nashenden Valley while the south-facing portal emerges at the base of Bluebell Hill. At the time of its completion, the tunnel was reportedly regarded as being one of the largest railway tunnels to be constructed in Europe as well as the largest and deepest tunnel in the UK, possessing a length of 3.2km and an excavated cross section of up to 174m (13m diameter). A key reason for the larger than normal cross section was a result of environmental sensitivity having prevented the construction of any conventional pressure relief shafts for the tunnel, thus necessitating alternative means for handling the pressure waves generated by high speed trains.