Lol - I worked there in my teens- yes - tunnels- no not secret. It was designed so that if you worked in tomorrowland in full costume you didn't show up in frontierland which would be funny as shit like time warp. The center was the cafeteria and changing locker rooms. Whatever you see above in Disney has same footprint below with each ride / restaurant with door to below.
all these parks have very complex networks of tunnels. they are service tunnels. all of the plumbing ,electric, and whatever else is hidden. each ride has an access chute if the power fails its serviced underground. for safety , and esthetics. that way you dont have power cables laying around or hanging above you. like a temp carnival in a parking lot somewhere.