One of the great things about art can be its ability to open doors. A few years ago, for example, Battersea Power Station was opened to the public ostensibly for an art exhibition, but in reality it gave curious Londoners the chance to swarm around inside a derelict old building they thought they'd never be able to explore. For the next month there's a similar opportunity to take a look inside a long-locked echo of London past - the Kingsway Tram Tunnel. And this time the excuse is knitting.
Between 1908 and 1952, any London commuter could have ventured beneath the streets of Holborn for the price of a tram ticket. Steps led down from Kingsway to an elegant subterranean station with a central island platform, from which trams departed regularly towards Aldwych to the south and Angel to the north. Initially designed for single-decker vehicles only, the tunnel was later deepened to allow the passage of double deckers. But even these couldn't remain profitable when faced with competition by car, bus and tube, and eventually the ramps down to this lamplit underworld interchange were firmly sealed. If this all sounds impossibly romantic, rest assured that the modern reality is rather gloomier. All you need is a booking for the exhibition and you can find out for yourself.
The arty installation is called Chord, and it's free, and it runs hourly from the junction of Southampton Row and Theobald's Road. Make yourself known to the guide with a clipboard and he or she will unlock the very obvious gates at the top of theramp and lead your group down into the rarely-seen depths. So many times I've walked past and looked through the railings and wondered what it might be like down there. And now I know.
The Kingsway tunnel is dark (as you might expect) and damp (probably thanks to Wednesday's downpour) and also unexpectedly long. This is no brief underpass, this is a proper underground carriageway with double tracks and a ribbed metal roof. Various building materials are stashed away in adjacent alcoves as the tunnel dips and rises towards a cavernous pillared space that was once Holborn Tramway Station. The retro feel here isn't entirely genuine - the tunnel's been used several times for filming so the fading posters and roundels on the walls are mere set dressing. Daylight creeps in down two steep confined staircases, while the sound of snarling traffic is ever audible from the roadway above. Rest assured, your guide will leave you free to explore this section of the tunnel (ooh, click, ooh, flash, click) while they wander on ahead to switch on the machines.
Ah yes, the artwork. In the far reaches of the tunnel are two giant (what can I possibly call them, erm...) swirly yarn turbines, probably located somewhere underneath the traffic lights at the end of Great Queen Street. Each has three arms each with another three arms, a bit like a whirling fairground ride, and positioned on each are six reels of brightly coloured string. These very slowly rotate and move apart, twisting a technicolour rope between them that grows at a rate of 20cm per hour. Yesterday the rope was about 20 metres long, which suggests that artist Conrad Shawcross had been playing with his machine for quite some time before the public arrived. I didn't notice anything growing while I was there, but over the month of the exhibition the cable should extend to fill most of the tunnel below ground. It creates, of course, a "strong structural metaphor" as "a clear linear entity made up and formed by a cyclical process". Because that's art. And we wouldn't be down here without the art.
30 minutes below ground, for free, in a long-lost London Transport tunnel, photos permitted, with a mechanical Heath Robinson rope-weaver - what's not to love? Book now, or else you may forever wonder what lies beneath.