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The North Downs Tunnel is the name of the tunnel that carries the Channel Tunnel Rail Link through the North Downs, at Blue Bell Hill in Kent. The reinforced concrete tunnel was completed in December 2001 as part of phase 1 of the CTRL, £5 million under budget and 5 months ahead of schedule. Over 100 workers working 24 hours a day made this possible. All this despite the site being percieved as potentially a major risk before the project.

It is a 3.2 km (2 mile) long and, 12 m (40 ft) diameter and descending to 100m below the chalk hills. This made it the largest and deepest twin track railway tunnel ever constructed in the UK. The high speed trains can reach speeds of 300kmh (186mph) though in everyday practice the tend to travel slower through the tunnel and the crossing at the Medway Viaduct over the River Medway for safety reasons.