The Dartford Cable Tunnel is a tunnel upstream of the Dartford Crossing built in 2003–4. Its diameter is ~3m. It is designed to carry and allow for maintenance of 400kV national electrical grid cable beneath the Thames; it is accessible by foot as a crossing of the Thames, but by authorised personnel only. The Queen Elizabeth II Bridge The Dartford Crossing joins Dartford and Thurrock across the River Thames and effectively forms part of Londons orbital M25 motorway. ... 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... 2004 (MMIV) was a leap year starting on Thursday of the Gregorian calendar. ... DIAMETER is an AAA protocol (Authentication, Authorization and Accounting) succeeding its predecessor RADIUS. // The name is a pun on the RADIUS protocol, which is the predecessor (a diameter is twice the radius). ... The metre (or meter, see spelling differences) is a measure of length. ... The volt is the SI derived unit for electric potential and voltage (derived from the ampere and watt). ... The National Grid is the high-voltage electric power transmission network in Great Britain, connecting power stations and major substations and ensuring that electricity generated anywhere in Great Britain can be used to satisfy demand elsewhere. ... The Thames (pronounced //) is a river flowing through southern England, and one of the major waterways in England. ...