Thorin the Old, his grandfather lived under in the Lonely Mountain. Surrounded by riches and treasures, they grew rich and famous. Dale a neighboring town also grew rich with trade, mostly because of its location. This is what brought Smaug, the dragon. He destroyed everything in Dale, burning buildings. Killed armies,towns,people, and kingdoms, Smaug eventually layed to rest under the hills of gold he buried himself in, and will stay unless killed or dies (which is mostly forever). Thorin being lucky he was a traveler, was out exploring the lands when he found his parents with grim looks. Asking how they left unharmed they immedatialy ignored him. Making a living off low level blacksmithing they long thought of there gold that lyes underneath Smaug. After some time Gandalf went looking for THorins father and found he had been carrying a key and map of the mountain. And now giving to Thorin, the rightful heir.