The M25 famously passes underneath a cricket pitch on the edge of Epping Forest, where the need to preserve sporting status quo demanded the creation of the Bell Tunnel. But another (longer) tunnel exists to the north of London, on top of which you can play basketball, pick fruit or even have a picnic (so long as you've checked the grass really carefully first).
The Holmesdale Tunnel carries the M25 through the built-up neck of the Lea Valley. The planners of Ringway D identified a thin strip of land where Waltham Cross touches London where they could drive through a six lane motorway with a minimum of demolition. The area had once been full of glasshouses for market gardening, but after the war these were mostly replaced by housing, leaving a green space locals nicknamed the Backfield. It divided Cameron Drive in Waltham Cross from Holmesdale in Enfield, with houses in the latter facing out across the grass. Many children used the Backfield as a playground, with camping and dogwalking also popular pursuits. Right, we'll have that, said the engineers, and they made plans to dig it up.
The Holmesdale Tunnel would be 670 metres long, linking M25 junction 25 to a viaduct over the railway and the River Lea. It would be a super-underpass, built to motorway standards, and constructed using cut and cover techniques. There'd be room for three lanes in each direction, plus a raised walkway either side for evacuation purposes, plus a central wall to keep the two carriageways apart. Once the trench was completed a concrete slab was laid across the top and then covered with topsoil, allowing a new recreational space to be created. When completed in 1983 it was the most expensive section of road ever built in Britain with a price tag of almost £30m. If you'd like to read more about the technical intricacies of its construction, this four-page article from the July 1982 edition of Ground Engineering magazine will tell you more than you ever need to know.
Millions of vehicles have passed through the tunnel since it opened, but local residents see none of that, only the Holmesdale Tunnel Open Space. It's mostly grass, and mostly featureless, but there is a multi-purpose sports pitch up one end with basketball hoops and goals for football, plus a wildflower area beyond. At the other end is a brick building owned by the Department of Transport which houses the ventilation system and a pumping system for removing groundwater. Trees can't be planted because the topsoil is too thin, but a short line of fruit trees has been added along one edge, and a couple of footpaths wend across the centre to link the two sides.
The old Backfield site is still the boundary between Hertfordshire and London, with the motorway and everything above it in Enfield and the houses to the north in Broxbourne. It's also still technically private government land, "to which the public is permitted to have access". A Friends group exists to organise kickabouts for kids and help keep the Open Space in order, but there are several hints that this may be in decline. It's been three years since they last tweeted anything, their webpage has expired, and it seems their last meeting in August had to be cancelled. The golden years of replanting the fruit trees after they were "destroyed by Dogs jumping up and ripping the the branches off" appear to be long gone.
There have been a couple of important upgrades recently, however, one down below and one up top. The motorway has been widened by removing the raised walkways, which allowed a merge lane to be added to/from the J25 roundabout, removing a two-lane bottleneck for through traffic. Alternative emergency access points were provided in the tunnel in the gap between the two carriageways, and the life-expired ventilation system was upgraded for good measure. This keeps all the motorists happy.
Meanwhile at the eastern end of the open space, close to the portal where the traffic rushes out past the Tesco Distribution Centre, an arty Gateway feature has been created to mark the point on the High Street where two boroughs meet. Several 'alphabet cubes' have been scattered across the pavement, which read ENFIELD from one direction and BROXBOURNE from the other. A bee-friendly garden with raised planting areas has been created between parallel gabions, plus an actual bench to sit on, which is a bit of a departure around here. There's also a mysterious churchlike sculpture, tiled and ideal for clambering, but with no hint as to precisely what it represents.
What I liked most about the Holmesdale Tunnel, other than the surreality of an invisible motorway, were the information boards scattered along its length explaining the site's past and present. I'd never have been inspired to come home and write all this otherwise, indeed much of the backstory would have passed me by. Next time you're orbiting London by car give a thought for the community you're ducking under, dead ordinary but spared the wrecker's ball, and still playing basketball on the roof a third of a century later.