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 2003âÂÂ04 at a cost of ã11 million (equivalent to ã million in ). The tunnel plays a crucial role in maintaining electrical supply infrastructure beneath the Thames, ensuring reliable power transmission.