The Hobbit - Chapter 11 - On The Doorstep
As they approach the foothills of the Lonely Mountain, the land turns bleak and barren. All greenery and other living foliage have been burnt away by Smaug. When they reach the foot of the mountain, Bilbo and three dwarves are sent to investigate the main entrance on the south side. The entrance looks far too dangerous — it is the gate that Smaug uses — so the company decides to search out the secret door described on their map, which is on the west side of the mountain.
After hours of searching, Bilbo finally locates a narrow passage along a cliff that leads to a flat, smooth patch on the mountain’s side. Though the patch must be the door, the dwarves cannot find a way to open it, as they have forgotten the message that Elrond read from the map. The dwarves bang at the door with picks and axes but to no avail. They grow discouraged.
One evening, Bilbo is sitting outside the door, lost in thought, when a thrush lands nearby and begins to knock a snail against a stone with its beak. Suddenly, the hobbit remembers the riddle on the map. He quickly gathers the other dwarves by the door, and they watch as the sun slowly sets. With the sun’s last light, a single ray falls on a part of the door, and there a rock falls away to reveal a keyhole. Thorin quickly takes the key that came with the map and places it in the rock — when he turns it, the door’s outlines appear. The dwarves and the hobbit push open the door and stare into the depths of the mountain before them.