The graphic above shows the longest examples for each type of tunnel. See below for more details.
Longest tunnel of any kind
Little-known fact: London contains the longest tunnel in the UK, longer even than the channel tunnel. You may well use it every day without realising. It is the Thames Water Ring Main, a looping circuit of tap water that runs for approximately 80km (50 miles) beneath London. With an average diameter of 2.5 metres, this is a proper tunnel rather than a mere water pipe, though you'd need an aqualung to traverse it.
The main section was constructed between 1989 and 1995, and future extensions may push its length still further. One of the few visible signs can be found on the roundabout between Shepherd's Bush and Holland Park, in the shape of the surge tower shown above.
Longest tube tunnel
Did you know that most of the Underground network is actually overground? Only 45% of track runs through tunnels. The longest on the network is the Northern Line tunnel between East Finchley and Morden, clocking in at 27.8km (17.3 miles). The route was carved out in stages between 1890 and 1939, and was almost certainly the longest tunnel in the world at the time.
A well known sculpture of an archer by Eric Aumonier guards East Finchley station, his bow pointing towards the tunnel portal. It's often written that his unleashed arrow could once be found in Morden, though this appears to be an urban myth.
The second longest tube tunnel, in case you're wondering, is the Victoria line (22km, 13.7 miles) then the Piccadilly line between Bounds Green and Barons Court at 19.6km (12.2 miles).
Longest Crossrail tunnel
When it opens in 2018, the Elizabeth Line (or The Purp, as we're calling it, after its line colour) will include 42km (26 miles) of new tunnel, or 21km in each direction. It's not entirely continuous however, with a couple of gasps for air through the Royal Docks. So what will be the longest stretch of unbroken tunnel? Crossrail doesn't explicitly say, but we can work it out from the individual 'tunnel drives' used to construct the line.
Royal Oak to Farringdon: 6.8km
Farringdon to Limmo: 8.3km
Limmo to Victoria Dock portal: 0.9km
This gives a total of 16km (9.9 miles) for the tunnel between Royal Oak and Victoria Dock, which is marginally longer than the route from Royal Oak to Pudding Mill Lane.
Longest private rail tunnel
There's only one contender here. The disused Mail Rail tunnel from Whitechapel to Paddington traverses 10.5km (6.5 miles) of tunnel. The line carried parcels across London for many decades before closing in 2003. A short section beneath Clerkenwell will shortly reopen as part of a museum experience.
Longest rail tunnel
The High Speed 1 link between London and the Channel Tunnel includes London's two longest rail tunnels (other than those on the tube network). One runs 10.5km (6.5 miles) from Dagenham to Stratford; the other takes trains the additional 7.5km (4.7 miles) from Stratford to St Pancras. But for a brief bit of daylight at Stratford International, this would be a continuous run of 18km.
Before construction of HS1, the Sydenham Hill tunnel, also known as the Penge tunnel, held the record at almost 2km (1.2 miles).
Longest road tunnel
The Limehouse Link passes beneath Limehouse Basin, hooking up the Isle of Dogs to the Highway and the City of London. Its tunnel runs to 1.8km (1.1 miles). Built between 1989 and 1993 at a cost of £293m, it remains one of the most expensive roads per metre in the world. The Rotherhithe Tunnel, with its long approach ramps, is in second place at 1.48km. Heathrow Airside Road Tunnel, a private route beneath the airport, is close behind at 1.42km.
The Dartford tunnel measures 1.43km, but this is just outside London.
Longest foot tunnel
Technically, the Rotherhithe Tunnel wins this category, as much of its length can be traversed on foot. However, you'd have to be a loon. If we consider pedestrian-only tunnels, then the Woolwich foot tunnel is the champ, measuring 498m. Its more-famous sibling tunnel at Greenwich is a mere 370m.
Longest canal tunnel
Anyone who's walked the Regent's Canal will have encountered the lengthy stretch of non-canal through part of Islington. The Angel area rests on a sizeable rise. Rather than sending boaters through an interminable flight of locks, the canal's engineers instead plumped for an 878m tunnel. It's only wide enough for one narrowboat, so the tunnel must have been something of a bottleneck in days of yore.
Longest electricity tunnel
One category of catacomb that often gets missed: the many cable tunnels beneath London. The biggest of all are currently under construction. National Grid's London Power Tunnels will eventually stretch 32km across the capital. The longest section is the Hackney to St John's Wood tunnel, which is 12.5km. We recently took a look down there.
The network of sewers beneath London all intermingle, so you could theoretically walk for hundreds of kilometres without retracing your steps. In terms of distinct sewers, several arms of Joseph Bazalgette's interceptor system boast around 19km (12 miles) of linear feculence. If built, the planned Thames Tideway interceptor tunnel will follow the course of the river for 25km (16 miles).
Longest tunnel slide
And finally... London's most sinuous attraction has opened, a helical slide from the 74 metre viewing platform of the Orbit Tower. The twirly fun-tunnel runs for 178 metres, supposedly making it the longest tunnel slide in the world. It takes 40 seconds to ride. How to get tickets.
All photos by the author. Orbit illustration by Bblur Architects.
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