This opened up quite a few months ago but ive been holding back for no real good reason.
Basicall Banksy (England's best known graffiti artist) was let loose on a now out of use tunnel at Waterloo station. I tried to get in while the exhibition was properly open but there was a que for the que so we flagged it. This site has some good shots and a vid of what it was like.
I went back the week after and took phots of my own which you can see HERE.
Over the cause of the exhibition you were allowed to put up your own stuff which is the section out in daylight.