The ancestral home of the King under the Mountain, it was captured from Thrór by Smaug the dragon, but later reclaimed by Dain after the Battle of Five Armies.
In The Hobbit, Bilbo and Thorin's company travel to the Lonely Mountain to regain the treasure Smaug had stolen. Set into the side of the mountain was a secret door, five feet high and wide enough for three to walk abreast. Gandalf had managed to obtain the door's key, which would only open it when the setting sun and the last moon of autumn were in the sky together. By a fortunate coincidence, this happened just as Bilbo and the Dwarves arrived.