From 10:30 today it will be revealed on http://www.thecansfestival.com/  where you can participate in some stencil graffiti this weekend .

The tunnel has already been decorated by several artists including Banksy. 
  

http://en.wikipedia.org/wiki/Banksy

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html