A gleam of light came straight through the opening into the bay and fell on the smooth rock-face. There was a loud crack. A flake of rock split from the wall and fell. A hole appeared suddenly about three feet from the ground.

     Quickly, trembling lest the chance should fade, the dwarves rushed to the rock and pushed – in vain.

     “The key! The key!” cried Bilbo. “Where is Thorin?”

     Thorin hurried up.

     “The key!” shouted Bilbo. “The key that went with the map! Try it now while there is still time!”

     Then Thorin stepped up and drew the key on its chain from round his neck. He put it to the hole. It fitted and it turned! Snap! The gleam went out, the sun sank, the moon was gone, and evening sprang into the sky.

     Now they all pushed together, and slowly a part of the rock-wall gave way. Long straight cracks appeared and widened. A door five feet high and three feet wide was outlined, and slowly without a sound swung inwards. It seemed as if darkness flowed out like a vapour from the hole in the mountain-side, and deep darkness in which nothing could be seen lay before their eyes, a yawning mouth leading in and down. 

– J.R.R. Tolkien, The Hobbit,
Chapter 11