"Hidden Mickeys started out as inside jokes among the Walt Disney Imagineers. A Hidden Mickey is an image of Mickey Mouse concealed in the design of a Disney attraction (ride, resort, etc...). Originally, it took the shape of a head and ears silhouette, one large circle with two smaller circled on top in the appropriate place, but Hidden Mickeys can take on many forms..."

Again, I'll bet I was the only person who didn't know about this until just a few minutes ago.