Hello Claudie,
I think you are referring to
"Disney's Key to the Kingdom Tour". This tour is 4 1/2-5 hours in length, with lunch included and just in case you are not aware, you may not bring a camera. Many backstage places are visited during this tour and no one is allowed to take any pictures here. Believe it or not, the tunnels you are referring to are called "Utilidors" and I have researched this quite a lot, I must tell you that it is such an interesting subject for me...I think you will like it.
A sneak peak behind the meaning and importance of "Utilidors":
Hidden from the view of the thousands of Disney World visitors is a labyrinth of out-of-sight tunnels called Utilidors. The term is taken from the words, "utility corridors," which is a conduit through which utility lines such as sewer, electricity, or water are carried. Of course, the concept can be (and is) expanded to facilitate the movement of many other things - even people. The Utilidors under the Magic Kingdom are built on nine acres, take up 392,040 square feet of space, and are filled with action. The floor plan resembles a wheel with spokes, the hub of which lies beneath Cinderella's Castle. From there, the tunnels branch out to all the other "lands." The original idea of a system of tunnels stemmed from the fact that Walt Disney was adamant that the integrity of each land "theme" be preserved. He did not like the idea of a youngster seeing someone dressed in a Frontierland costume walking through Tomorrowland, which would spoil the magic. Hence the Utilidors were designed from the outset. Walt Disney did not dig tunnels under the Magic Kingdom. The fifteen-foot-high corridors were built at ground level!!!
These are some fun facts you might have wanted to know, I find this stuff intriguing myself. Have a magical day!!
Brigitta