I'd heard a slight variation on the underground element to Disneyland/ world. I have it from a friend (not an employee of the Disney organisation, but a postgrad scholar of American Studies who claims to have to know about these things...) that once in costume, Mickey and friends are not allowed to break character in front of the public. So keen is the company to maintain this element of fantasy that the rest-rooms where the costumed employees may remove their mouse heads (or whatever) and enjoy a quick chat or cigarette are located in a bunker-like basement level of the staff buildings with no natural light.
Anyone found breaking character by taking their mouse (or duck, or whatever Goofy is...) head off or scratching their bum or swearing anywhere where the public might possibly see or hear them is dismissed for breach of contract.
Strange to note that most of the tunnel stories we've mentioned (My initial Keele Woods one, the institutional one in Wisconsin, and my firiend's account of the Disney one) all have a punitive element to them. The Keele story is made more thrilling by the 'fact' that you can get instantly sent down for snooping around; in the Wisconsin case MercuryCrest details, employees are discouraged from discussing the tunnels or asking questions; and in my American Studies friend's embellishment of the underground Disney story described initially by liveinabin, the threat of the sack looms over any anthropoid mice who transgress the laws concerning the underground mouse-lair.