The tunnel is 3.2 kilometres (2 mi) long, with an internal diameter of 12 metres (40 ft) and a cross-sectional area (CSA) of 150 square metres, and descends to a depth of 100 m (325 ft) below the chalk hills. Thus, at the time of its completion, the North Downs Tunnel was both the largest (in terms of CSA) and deepest twin-track railway tunnels to have ever been constructed in the UK. Trains using High Speed 1 are able to attain speeds of up to 300 kilometres per hour (186 mph) whilst in the tunnel.