Erebor

Erebor was a city inside a mountain later known as Lonely Mountain with the city of Dale. Both were cities inhabited by dwarves. The dwarves built the city after they were living in a town called Khazad-Dum (also called Moria) when a balrog appeared and they had to leave Khazad-Dum. The dwarves at Erebor became rich and all was well until Smaug, a dragon, came and destroyed the cities and stole all their gems and other treasures. The mountain would then get the name of Lonely Mountain. Later Bard, the king of Dale, would kill Smaug and share the treasures with the elves of Mirkwood.

Source: "The Hobbit Movie-Places:Erebor" http://derhobbit-film.de/erebor.shtml