A strange and magical place, which typically can only be reached through either a combination of large amounts of shrooms and the use of portable toilets, or prolonged exposure to the transcendent music of the band Phish. Phish guitarist Trey Anastasio was the first to describe the back of the worm; during a show in Amsterdam in 1997, in the middle of a creepy, ambient jam called Wormtown, Anastasio described to the audience his experience of being sucked into a public urinal and riding around on the back of giant worms. Very few others have been lucky enough to experience The Back of the Worm, but most of the people who have did so at Phish concerts.
I think you know where you are. ...YOU'RE ON THE BACK OF THE WORM! Seriously, though, stay away from those portable toilets in Amsterdam.