This is very cool. ; I have an uncle who is retired US military, and he's taken on a second career with Disney (been there 20 years now). He's always told us about the tunnels, and how employees/ cast members can get to where they need to be - it's also key for characters especially - e.g., Mulan will never show up in Tomorrowland (the video talks about the Walt's determination after DL experience.)
One fact I picked up this past trip from my uncle, that I didn't know about, is that Epcot has a minor underground (well - after watching the video talk about the swamp, I guess main floor) as well. ; It's no where near as extensive as MK, and is just in the main hub at the entrance - but there is some stuff there too.
The video is very cool. ; Going to send that to some clients.