What's London's Longest Tunnel?

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By M@ Last edited 17 months ago
What's London's Longest Tunnel?

The graphic above shows the longest examples for each type of tunnel. See below for more details.

Longest tunnel of any kind

A water shaft silhouetted
Image Matt Brown

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

Silhouette of an archery sculpture against a twilight sky
The archer who guards the northern portal at East Finchley. Image Matt Brown

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/Elizabeth line tunnel

A map showing the different tunnelling sections of the Elizabeth line, shown in different colours
Graphic showing the different tunnel sections as bored. Image courtesy Crossrail.

The Elizabeth Line includes 42km (26 miles) of new tunnels, or 21km in each direction. It's not entirely continuous however, with a couple of gasps for air through the Royal Docks.

The longest stretch of tunnel is the section that runs from Royal Oak in the west to Victoria Dock at 16km (9.9 miles). Once trains run through to Shenfield (November 2022), the alternative measure from Royal Oak to Pudding Mill Lane will be marginally shorter.

Longest private rail tunnel

A long and sinuous tunnel with a rail track
Image Matt Brown

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

A long, long tunnel with white tile walls
Image Matt Brown

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

A tunnel opening on a canal with a brick bridge spanning the water
Image Matt Brown

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 878 m tunnel.

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.

Longest sewer

A filthy brown puddle of excrement in a sewer
The floor of the Fleet sewer. Image Matt Brown

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

The curly grey slide along the red Orbit tower structure
Image Matt Brown

And finally... London's most sinuous attraction, 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.

Like this? Find out how deep London goes, with this graphic.

Last Updated 03 October 2022