Although they legally belonged to the King of Gondolin, the mines were used and controlled by smithies loyal to Maeglin and his house. The mines were the source of the ore for many precious metals such as gold and iron. The mines lay approximately twenty-five miles from the city at an unknown depth under the Encircling Mountains.
It is unknown whether the mines existed before the arrival of Maeglin. When he was fully established, the mines seemed to come under his control and that of his band of smithies. There was forged a storehouse of weapons and other items. Everything including the mines perished with the rest of the Beleriand at the conclusion of the War of Wrath.