I don't think that the utilidors are under every square foot of the park.
What abou the belt/escalator ride from the end of the ride to the exit? That definately goes up, and ends at ground level. So SM must be somewhat underground.
I agree with I'mnoPrince.. but just from my experience of seeing the attraction with the lights on, it doesn't appear to go 'underground' in the sense that there is a tunnel or anything like that. The floor of the attraction might be 12-24 inches lower than the 'ground' level of MK exterior to the attraction but that's hardly what I would call a basement or 'underground'.
This map of the utilidor system seems to show the utilidors would go roughly underneath at least part of the Space Mountain structure but then its hard to say for sure.
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I had also planned to have all track above ground like WDW's ride (Disneyland's has 17 feet of track underground). With that in mind, this would bring the total height of my Space Mountain ride building to 134 feet. Since I had chosen the Disneyland Paris exterior, and their ride building is 141 feet-- only 7 feet higher, I decided to make the height of my Space Mountain 141 feet, the same as theirs; since this would make it a lot easier for the actual design, and would greatly help avoid distortion of the ride building by cutting out a few extra feet!