Disney's Magic Kingdom is the most popular theme park in the world, having attracted some 17.5 million guests in 2012. The vast majority of those visitors are completely unaware that a hidden underground complex lies beneath their feet. Most Disney fans (and Theme Park Tourist readers) will know about the "Utilidor" tunnels that are located underneath the park - but even they may not realize the staggering scale and scope of this subterranean world.
The underground tunnels span an incredible 392,040 square feet. They span most of the park, with the exception of the part of New Fantasyland that was previously Mickey's Toontown, as this was added as an expansion in 1988. You can get an impression of their scale by looking at the (possibly outdated) map below: