The Cellar was large, with a length of 180 feet and had two great doors leading up to a stairway to the Chamber of Thrór. It was known to have bats in it at least by the time of Smaug's life there.
The hall was delved and built by the Dwarves at an unknown time, possibly between TA 1999 and TA 2770 when the Dwarves of Durin's Folk ruled there. It is possible that the Bottommost Cellar was where the Dwarves mined the ore to make their crafts and stored them. After Smaug attacked and conquered the mountain, he gathered up all the Dwarves’ treasures and slept on them for over one-hundred and seventy years. On October 30 and November 1, TA 2941, the hobbit Bilbo on his travels with the Dwarves of Thorin and Company confronted Smaug with riddles until he got angry and unleashed his fiery blast on him, forcing him back up the Secret Tunnel on fire. Later when it was discovered that Smaug had found them out, Thorin and Company was forced down the tunnel into the Cellar and trapped inside when Smaug smashed the Side-door to rumble. Once inside, the Dwarves lit torches that illuminated a hall full of the ancient treasures and weapons of the Kings under the Mountain. Here the Dwarves basked in their finds and armed themselves and was where Bilbo found the Arkenstone and where Thorin gave him his Mithril Shirt. The company soon decided to leave the Cellar and climb the steps up to the Chamber of Thrór.