Yep, there's a huge underground cavern underneath the MK that not only houses our apartments, but also our shopping malls (300 stores, but they all sell the same plush items and t-shirts), private amusement park (it's where we moved 20,000 Leagues Under the Sea, Alien Encounter and Mr. Toad's Wild Ride), Cast Member Brainwash-and-Dry, and best of all, the Secret Lair for World Domination. My favorite location down there in Utilidoorville is the Michael Eisner Wading Pool -- it's full of money Bob Iger rolls around in when he comes to visit.
If you ever get lucky enough to go down there, ask to see the Audio-Animatronic of Walt Disney that Roy Disney had built to dispense brotherly advice and backrubs.