From what I've heard, (and I know Legacy is just waiting to pounce on this if I'm wrong
), the tunnel is actually encircling a whole portion of the house dubbed "The Island". It's excessively long but I believe that's because it's the only thing that A&D could've fit in that space with the scenes they wanted done.
Well actually, they might've been able to do different (read: better) scenes than they did and made the loop run a little shorter as a result but whatcha gonna do?