There's a horror story or three, maybe an scp, that is about that.

I dated a cast member so i more than once encountered mickey at home (there were like four of them sharing an apartment. anaheim sucks)

There are custom Disney-themed FNAF ripoffs

The distinction between "the tunnels are underground" and "the tunnels are above ground, it's just that the stuff visitors walk on is then above that" seems like a pretty subtle one.

All that says is that Disney moved the ground up so that they could have underground stuff without it getting flooded. Or is the entire park the roof of a one story building?

I've worked at Disney, the tunnels exist and can take you pretty much all over the park

https://en.wikipedia.org/wiki/Disney_utilidor_system

so I suspect that the tunnels are limited if they exist at all.

They're not top secret, but you've guessed right all along.

It's more of a mixture of tunnels, backstage corridors between attractions, and open areas within internal attraction-voids.

As in, there really are tunnels, but no one with any knowledge on the subject never claimed the whole thing was tunnels.

EDIT: Actually, you're both right in parts because in large portions also have the whole elevated by one level thing, and the "outside" is really the second story. I skimmed that critical part on the first look at the wiki.