The Gum Wall is a brick wall covered in used chewing gum located in an alleyway in Post Alley under Pike Place Market in Downtown Seattle. Parts of the wall are covered several inches thick, 15 feet (4.6 m) high along a 50-foot-long (15 m) section. The Post-It notes were seen on a wall on the way to the Gum Wall. This is, no doubt, a Lennon Wall.

I, of course, contributed to the artwork by sticking a wad of bubble gum on the wall...though didn't touch the wall. :)