It consists of a tunnel (for northbound traffic) and the Queen Elizabeth II bridge (southbound). In all but name it's the bit where the M25 vaults over, or dodges under, the Thames Estuary, linking Thurrock in Essex with Dartford in Kent.
Neither the tunnel nor the bridge were built for bikes. However, thanks to paragraph 27 of the Dartford-Thurrock Crossing Act 1988, cyclists have to be transported free of charge.
(A reliable source tells me this was inserted by our bicycling baronet chums in the Lords, after the evil anti-cycling House of Commons tried to push it through without pedalling provision.)
In practice this means you cycle to a control-point car park at either end, and stand around until a chap in a tie and high-vis jacket pops out and asks you if you want a lift across. A lurid, op-art Land Rover suggestive of a Zandra Rhodes migraine turns up; your bike goes on a rack on the back and you're whisked across.
The facility works 24 hours a day, seven days a week. Each Land Rover can take up to three people at a time. They have a big bike trailer for cycling groups, though they need a phone call in advance to set it up.
On both my north and south trips I had a friendly and chatty lady driver whose knowledge of the relative merits of Bluewater and Lakeside shopping centres, on opposite sides of the estuary, was detailed and comprehensive. Up on the Thurrock side I had a couple of pints in a rather shabby Essex pub populated by Fast Show characters where everything was either sticky, broken, or nailed down.
The southbound journey, across the bridge, is impressive. You're pretty high up (180 feet, my likeable cabbie informed me in between observations on the new John Lewis food court in Bluewater) and get a commanding view of the refineries, ships and marshy sweeps. Splendid, in a Netherlands sort of way.
But being in a vehicle, you can't stop, and it all flits superficially by, a glimpsed estuarial zoetrope. If only they'd built a bike lane... like they did with the Humber Bridge, which is much better because you can therefore dawdle and stop and enjoy the panoramas of mud, sand and chimneys.
Neither bike access point is easy to find by bike. Signage is sporadic and the route not obvious. Your final mile or two will be spent in narrow strips of tarmac alongside what are effectively motorways. The most convenient way to do the crossing from London is to cycle out on National Cycle Route 1, which runs virtually all along the south bank of the Thames from the centre to Dartford. (Reckon on 20 miles/30 km taking about four hours.) There are regular trains back from Dartford to London Bridge/ Waterloo/ Charing Cross (40 minutes).
And doing it all by bike has a great advantage: there is no chance of you following the siren calls to Bluewater or Lakeside and being suckered into buying a sofa.