This weekend's bank holiday in London is almost over, so you better hurry if you want to have a chance to compete with elusive graffiti artist, Banksy. The Cans Festival opened up a half mile of unused tunnel behind Waterloo Station to house some original Banksy stencils, along with other invited street artists. The goal, in Banksy's words, is to "transform a dark forgotten pit into an oasis of beautiful art." Already visitors to the exhibition have added their scribblings to the walls. Although we have a feeling it will turn into spaghetti of mixed messages, the bigger danger is a word of caution posted on Cans’ Website: art outside the designated area may well result in prosecution.