Anaheim Disneyland has no tunnels to speak of. There are local underpasses, mostly under the berm (makes it easier to avoid getting run over by a train.)
When a new attraction building is constructed, there are basements, often confused with tunnels. No tunnels to the Island, no magical underground. In Tomorrowland, when the stage area was rebuilt, it made sense to drop in a small tunnel access, since they were pouring concrte foundation walls anyway.
Remember, DLR was built at grade lvel - on the ground. Orlando WDW is built up above the original grade, so the Utilidoor system is merely a system of rooms and halls in the basement, designed to fix the backstage-onstage access problems learned from DLR.
As for rides being "underground" - in relation to the entry level, this may seem to be so. In reality, rides are housed in backstage buildings that go into the ground a few feet, but not truely underground.
Space Mountain was recently excavated down a few more feet to accommodate the new track design, but again - no underground.
There are, however, two ancient caverns below the Matterhorn and Rivers of America, where Orcs, Dwarves, and a few Trolls live. They feed on the cats.