It has taken two years of tunnelling under London, but here’s the end result—Crossrail’s new ‘super tube’ deep under ground, ready to rock’n’roll by 2018.

Now Crossrail bosses are challenging the public to walk through the tunnels—before the trains start running, of course—in a promotion competition closing this Friday.

You can take a ‘selfie’ standing in front of a Crossrail’s ‘behind the hoarding’ posters by January 30 for a chance to win a guided tour of the new tunnels. There are just three days left!

The posters can be seen at several station sites, Whitechapel, Canary Wharf, Liverpool Street, Moorgate, Custom House, Farringdon, Bond Street or Paddington.

Or you can download the poster from the Crossrail website and take a selfie showing it in the background, if you can’t get to a station.

Meanwhile, a new exhibition of Crossrail’s tunnelling story opens at the London Trasnsport Museum in Covent Garden on Sunday, showing London’s hidden subterranean landscape.

The exhibition brings to life the sheer scale of the work being done to complete Crossrail in time for the first train arriving in three years’ time.