The Chiltern Tunnel is a high-speed railway tunnel currently under construction in Buckinghamshire and Hertfordshire, England, and will upon completion carry the High Speed 2 (HS2) railway line under the Chiltern Hills. The twin-bore tunnels, which are 16.04 km long, will be the longest on the HS2 line. Each tunnel will also have additional 220 m (720 ft) entry and 135 m (443 ft) exit perforated concrete portals to reduce sudden changes in air pressure and subsequent noise.