Ever been browsing satellite view and stumbled across something like this?
We're looking at a residential area between Alperton and Hanger Lane tube stations. But what is the long, green feature highlighted in red?
Our first assumption would be an abandoned railway, like the Parkland Walk in north London, or the Addiscombe Railway Park in Croydon. It's not, though. There's more to it.
The curious feature was first pointed out to us by Tarun Patel on the old Londonist Urban Oddities group. He reckons to have traced the linear path all the way up to Muswell Hill. Here's another section, about a half-mile west of the top image.
And here's a continuation, north-east in Neasden...
If we take a look on streetview, we can see that this isn't the route of a stream. It's a strip of green land that goes on for miles, with no public access. This view is from Dudden Hill Lane in the image above, but is representative of the feature as seen from many other roads along the route.
If you look closely, you can even see the line crossing diagonally over exclusive The Bishop's Avenue near Highgate. Along its route, we see empty patches of land in these vaunted acres of mansions and swimming pools.
How far does it stretch?
Having spent 20 years exploring every corner of London, we'd somehow never chanced across this epic landscape feature before. But once you spot it on the map, it's hard to miss. Patel has made a Google Map showing its route through north-west London (as he perceives it... we should make clear that the map was put together by scrutinising satellite view, and not through any official documentation, which doesn't seem to be publicly available).
So, if this sleuthing is right, the feature runs between the Fortis Green covered reservoir and the Kempton waterworks near the Thames.
So what is it?
Given the termini, you've probably guessed, correctly, that this is a water main. It was constructed at a shallow depth, before most of the housing came along. Rather than relay the pipeline at greater depth, it has instead been segregated off from the communities that have grown alongside it.
The pipe was constructed in the early years of the 20th century to carry water up from the Thames near Kempton to Cricklewood. From here, it was pumped further uphill to the reservoir at Fortis Green, where it was stored for later distribution across London. The 42-inch (1.07 m) pipe opened in 1908, as described on the Kempton Steam Museum website.
The pipe is still in use today, though London's water provision has grown hugely since 1908 to meet the rising population. The London Ring Main, a kind of M25 of fresh water, handles much of this demand.
A blooming good idea
Here and there, the route has been opened to carry a footpath, and occasionally a development has intruded over the line. But, by and large, the miles of route are fenced off to the public.
It might be unsuitable for general access, or too expensive to convert it into a footpath. But here's a simple and inexpensive idea: how about turning it the world's longest flower meadow?
Update: Thanks to Russ Garrett, who notes that part of the pipe is built into OpenStreetMap.