The Dartford Cable Tunnel is a 2.4 km utility tunnel beneath the Thames, upstream of the Dartford Crossing. With a diameter of ~, it carries a 400 kV National Grid electrical transmission cable. It is accessible by foot as a crossing of the Thames, but by authorised personnel only. It was built in 2003–04 at a cost of £11 million (equivalent to £ million in ). See also 400 kV Thames Crossing Thames Cable Tunnel Utility vault Crossings of the River Thames Tunnels underneath the River Thames References Infrastructure in London Dartford Tunnels underneath the River Thames Tunnels completed in 2004