The nurikabe (ぬりかべ) is a yōkai, or spirit, from Japanese folklore. It manifests as a wall that impedes or misdirects walking travelers at night. Trying to go around is futile as it extends itself forever. Knocking on the lower part of the wall makes it disappear.
Here's a Nurikabe sculpture and print by Andrew Bell:
And a work in progress shot:
You can buy the painting or a print at Bear and Bird Boutique.
From the same Wikipedia entry, here's a "statue depicting the nurikabe from GeGeGe no Kitaro by Shigeru Mizuki on the road bearing his name":
It's the perfect foil to my favorite god. And also the inspiration for Super Mario's Whomp: