Secret Tunnel

The Secret Tunnel was a long indoor tunnel leading to the Bottommost Cellar, the lair of Smaug of the Lonely Mountain (Erebor).

The tunnel was about five feet high as with the Side-door and ran about two miles and was entirely smooth and followed a gentle but never deviating downward slope, down to the Bottommost Cellar of the mountain realm.

History
The Secret Tunnel as with the Side-door and its key were created at an unknown time by Thrór and his son Thráin before the fall of the Lonely Mountain. Their reason for creating it was unknown but literature points toward a means of escape in case of siege or other emergency. Unused for 171 years, the tunnel was rediscovered along with the enchanted Side-door by Thorin and Company on Durin's Day on October 30, 2941 and the hobbit Bilbo was charged with exploring it. Traveling down the tunnel for hours, Bilbo conferred with himself as to the reason for him doing this and going on this adventure to begin with, and that the Tookish part of him was at work. Towards the end if it due to Smaug, the tunnel grew warmer and warmer until it became quite hot.

After Bilbo emerged from the tunnel with news of Smaug's anger, the dwarves feared he would find out where they were and kill them so they all entered the tunnel and sought refuge in the mountain. Smaug indeed did find were they came from and smashed against the mountain, destroying the Side-door and sealing Thorin and Company in the tunnel.