Hobo Nickels

Since at least the 18th century unknown and uncelebrated artists have carved miniature bas relief sculptures into the surfaces of coins. In the early 20th century the Buffalo nickel was introduced in the U.S., and this particular coin, which featured the portrait of an Indian with bold features, was minted using soft metal making it easier to deface and transform. With the flood of idle hands and unemployed artists resulting from the depression, the phenomenon of the Hobo Nickels was born. Here's a great series of images of nickels carved to reveal skulls - a particular sub genre of the art form. Hobo nickel carving remains a popular hobby today and it even has its own society.

Via Colossal Art & design.