In 2005, a strange little thing popped up outside a coffee shop in Ann Arbor, Michigan. It was a tiny doorway, just about the right size for, say, a small, mythical, flying creature. It was the first in what has become a series of "fairy doors" scattered about the city, hidden in discreet corners, sometimes leading to tiny, secret rooms.
Thought a couple of have reportedly vanished recently, a number remain as a sort of scavenger hunt of an attraction. Upon finding them, visitors have made a habit of leaving tiny gifts in the form of coins, candy or notes.
Messy Nessy Chic has more on the phenomenon, including its origin.