Here's a tidy little explanation of the "Utilidors" — the secret tunnels under the Magic Kingdom in Walt Disney World. I once got a tour of these, when I was working on Down and Out in the Magic Kingdom, which has some of its action there in the Stygian depths of the Disney utility corridors.
At the top of the map is the tunnel entrance. As a cast member, you park in a parking lot about 1 mile away from the tunnel entrance, and take a bus to the tunnels. If you're hungry, your first stop with be the "Mouseketeria." Burgers, pizza, sandwiches, and amazing omelettes are available for a fraction of the cost of the park prices. The "mouseketeria is immediatly on your right as you enter the tunnels from the bus (on the left in the map). If you look to the right of the entrance on the map, you will see costuming, and locker rooms. The best part of working in the Magic Kingdom was being able to show up in shorts and a t-shirt. You walk up to a window and say the name of the area or attraction you'll be working for that day, and your measurements, and they hand you a freshly laundered (questionable) costume. You change, and travel the utlilidor to your final destination.
Update: Jeff sez, "Just wanted to let you know that there is a high-resolution copy of the old Utilidor map available here. The one you linked in today's post is frustratingly small and hard to read."