What is a Hidden Mickey???

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

    i.e. one large circle with two smaller circled on top in the appropriate place, but Hidden Mickeys can take on many forms.

In designing, constructing or adding the final touches to an attraction, Imagineers subtly "hide" Mickey Mouse silhouettes in plain sight. Soon, it became a tradition, and as the word spread, Disney fans everywhere went on the search for Hidden Mickeys in Disney movies and theme parks.

So when you are at Disneyland -  keep an eye out for them.   

I will get you started:

   This Hidden Mickey is the laid out wheel cogs on the Thunder Mountain ride.