Lonely Mountain
In J. R. R. Tolkien's legendarium, the Lonely Mountain is a mountain northeast of Mirkwood. It is the location of the Dwarves' Kingdom under the Mountain and the town of Dale lies in a vale on its southern slopes.
In The Lord of the Rings, the mountain is called by the Sindarin name Erebor. The Lonely Mountain is the destination of the protagonists, including the titular Hobbit Bilbo Baggins in The Hobbit, and is the scene of the novel's climax.
The mountain is the goal of Bilbo's psychological quest in The Hobbit; scholars have noted that it and The Lord of the Rings are both structured as quests to a distant mountain, but that the quests have very different motivations. Further, the mountain is a symbol of adventure in The Hobbit, and of Bilbo's maturation as an individual, while to the Dwarves, it stands for the gain of beauty in return for loss of life.

Fictional mountain
Erebor stands hundreds of miles from the nearest mountain range. Tolkien's rendering of Thrór's map in The Hobbit shows it with six ridges stretching out from a central peak that was snowcapped well into spring. The whole mountain is some ten miles in diameter; it contains an immense wealth of gold and jewels.

Origins of the Kingdom under the Mountain
Erebor becomes the home of the Folk of Durin, a clan of Dwarves known as the Longbeards, after they are driven from their ancestral home of Khazad-dûm. In the latter days of the Third Age, this Kingdom under the Mountain holds one of the largest dwarvish treasure hoards in Middle-earth. Dale, a town of Men built between the two southern spurs of Erebor, grow in harmony with the dwarves. The Kingdom under the Mountain is founded by Thráin I the Old, who had discovered the Arkenstone there. His son, Thorin I, leaves the mountain with much of the Folk of Durin to live in the Ered Mithrin (Grey Mountains) on account of the great riches to be found in that range. After dragons plunder their hoards, the Longbeards, led now by Thrór, a descendant of Thorin, return to Erebor to take up the title King under the Mountain. Under Thrór's reign, Erebor becomes a great stronghold where the dwarves are numerous and prosperous.

Erebor in The Hobbit
In the Third Age, while the young Thorin II Oakenshield is out hunting, the dragon Smaug flies south from the Grey Mountains, kills all the dwarves he could find, and destroys the town of Dale. Smaug takes over the mountain, using the dwarves' hoard as a bed. King Thrór, his son Thráin II, and several companions escape death by a secret door. While Thrór and Thráin later perish, Thorin lives in exile in the Ered Luin, far to the west. On a journey, he meets the wizard Gandalf. Together they form a plan to reclaim the mountain. Gandalf insists that burglary is the best approach and recommends the hobbit Bilbo Baggins.Bilbo, Thorin, and Thorin's company of twelve other Dwarves travel to the Lonely Mountain to regain the treasure. They plan to use the secret door, whose key and map Gandalf had obtained from Thráin, whom he had found at the point of death in the pits of Dol Guldur. On Durin's Day, when the setting sun and the last moon of autumn are in the sky together, the day's last sunlight falls on the door and exposes its keyhole. The Hobbit enters the mountain and steals a golden cup.Smaug, enraged by the theft, emerges from the mountain and flies south to destroy Lake-town, which he suspects is the source of the "thieves". During this attack Smaug is killed by Bard the Bowman; Thorin claims the mountain on learning of Smaug's demise. However, the Men of Esgaroth, supported by Thranduil and the Elves of Mirkwood, march in force to the mountain to demand a part of the dragon's hoard as recompense for the destruction. Thorin, mad with greed, refuses all claims and sends word to his second cousin Dáin II Ironfoot, chief of the Dwarves of the Iron Hills, who bring reinforcements. Before battle can begin, an army of Orcs and Wargs descends on Erebor. Dwarves, Elves, and Men joine ranks against them, leading to the Battle of Five Armies. Thorin's nephews Fíli and Kíli are killed, and Thorin is mortally injured; he dies shortly afterwards. The title of King under the Mountain passes to Dáin.

Erebor in The Lord of the Rings
With the restoration of the Kingdom under the Mountain, the area becomes prosperous again. Dale is rebuilt under Bard's leadership, and Dwarves and Men reforge their friendship. Some of the Dwarves, led by Balin, leave Erebor to reclaim the ancient Dwarvish Kingdom of Moria. They established a colony there but five years later Balin is killed by an Orc, and soon afterwards Moria is overrun by Orcs and the rest of the Dwarves are killed. Gimli, a dwarf of Erebor and the son of Glóin, one of Thorin's twelve companions, is chosen to represent his people in the Fellowship of the Ring; he helps Aragorn regain the throne of Gondor.In the War of the Ring, an emissary from Sauron, the lord of Mordor, twice comes to Erebor and speaks to Dáin. The messenger asks for assistance in finding Bilbo Baggins and retrieving a stolen ring, and in return offers Moria and three of the seven Dwarf rings to Dáin, who declines to reply. Sauron's northern army, including many Easterlings, then attacks; Dale is overrun, and many Dwarves and Men take refuge in Erebor, which is promptly surrounded. Dáin is killed before the gates of Erebor defending the body of his fallen ally King Brand of Dale. Dáin's son Thorin III Stonehelm and King Bard II withstand the siege and rout Sauron's forces.

Analysis
Goal of psychological quest
The Jungian psychoanalyst Dorothy Matthews, viewing The Hobbit as a psychological quest, writes that the Lonely Mountain is an apt symbol of Bilbo's maturation as an individual, as the place where he takes on a leadership role and acts and makes decisions independently. The Tolkien scholar Jared Lobdell comments that he is "profoundly unsympathetic" to Matthews's approach, but that she "carries it off well". Lobdell explains, citing C. S. Lewis's essay "Psychoanalysis and Literary Criticism", that many different stories could, for instance, have the same Freudian interpretation, but be quite different as literature. He remarks on the other hand that a psychoanalytic approach is at least richer than a purely materialistic one.The scholar of children's literature William H. Green calls the Lonely Mountain the fourth and final stage of Bilbo's education. He identifies multiple parallels and repetitions of structure between the stages, each one involving a journey, privation, and "unlikely escape". The Lonely Mountain stage, too, symbolically echoes the first stage in the Shire: before setting out, Bilbo was peacefully smoking a pipe of tobacco at his own front door; at the mountain, the smoke is the dragon's, and its meaning is anything but peaceful. The Christian writer Joseph Pearce views the journey to the Lonely Mountain as a "pilgrimage of grace", a Christian bildungsroman, at its deepest level. Pearce states further that Bilbo's quest to the mountain parallels Frodo's quest to a different mountain, Mount Doom,  which he calls "a mirror of Everyman's journey through life".Two scholars of literature, Paul Kocher and Randel Helms analyse Bilbo's journey to the lonely mountain, describing it as the goal of his quest and the point at which it is achieved. Both compare the quest in The Hobbit with that of The Lord of the Rings, noting that the two novels, for all their differences including the reason for the quests, are structurally similar.

Gain and loss
The Tolkien scholar Tom Shippey notes that in The Hobbit, the lonely mountain is a symbol of adventure, and the "true end" of the story is the moment when Bilbo looks back from a high pass and sees "There far away was the Lonely Mountain on the edge of eyesight. On its highest peak snow yet unmelted was gleaming pale. 'So comes snow after fire, and even dragons have their ending!' said Bilbo, and he turned his back on his adventure."Amelia Harper, in the J. R. R. Tolkien Encyclopedia, writes that the mountain's history, as usual for the Dwarves, was a tale of "beauty gained and lives lost".

Adaptations
The Lonely Mountain: Lair of Smaug the Dragon is a board game produced in 1985 by Iron Crown Enterprises, designed by Coleman Charlton, which features groups of adventurers, either Dwarves, Elves, Orcs or Men entering Smaug's Lair to capture his treasure before he awakens."Erebor", specifically the southern spurs of the Mountain and Dale, is a playable map in The Lord of the Rings: The Battle for Middle-earth II. It has three gates, including the one Tolkien described and two which cannot be closed, to allow those playing as invading forces to easily enter the stronghold.The Lonely Mountain appears in Peter Jackson's film adaptations of The Hobbit:  An Unexpected Journey, The Desolation of Smaug, and The Battle of the Five Armies. The actual setting was Mount Ruapehu in New Zealand.

In astronomy
The International Astronomical Union names all mountains on Saturn's moon Titan after mountains in Tolkien's work.  In 2012, they named a mountain on Titan "Erebor Mons" after the Lonely Mountain.

References
Primary
Secondary
Sources
Tolkien, J. R. R. (1937). Douglas A. Anderson (ed.). The Annotated Hobbit. Boston: Houghton Mifflin (published 2002). ISBN 978-0-618-13470-0.
Tolkien, J. R. R. (1954a). The Fellowship of the Ring. The Lord of the Rings. Boston: Houghton Mifflin. OCLC 9552942.
Tolkien, J. R. R. (1955). The Return of the King. The Lord of the Rings. Boston: Houghton Mifflin. OCLC 519647821.