No harm in discussing it, I think... the rumor is probably still floating around out there. The first thing I thought is there a section of land from MK to Contemporary that is all at MK level? Obviously they wouldn't build a really for real underground tunnel; that's why the utilidors are built the way they are. The biggest thing working against this rumor, I think, is why even have a tunnel between MK and Contemporary in the first place? The utilidors have a purpose: to do your operations stuff out of sight of the guests and preserve the show in the MK. What would the point be of a tunnel between MK and Contemporary? If you need to move someone/something between the two, you just load up a van backstage at MK and take it to a loading dock at the Contemporary.
That would be a pretty long tunnel, so they would have to have a good reason and I don't see one.