Under construction tunnel

The mystery tunnel is found in Chip 'n Dale's. No one has ever, EVER seen what's inside it or heard what it's going to be. The most likely answer is related to the manholes released when bossbot HQ came out. Because the most likely place for Cog Nation is underground, that could be where it leads. But most importantly, we know it's something important because it's believed to be undergoing work for about two years, even during the making of Field offices. (field offices were NOT made 100% by Disney)