In the north of the world Melkor had in the ages past reared Ered Engrin, the Iron Mountains, as a fence to his citadel of Utumno; and they stood upon the borders of the regions of everlasting cold, in a great curve from east to west. Behind the walls of Ered Engrin in the west, where they bent back northwards, Melkor built another fortress, as a defence against assault that might come from Valinor; and when he came back to Middle-earth, as has been told, he took up his abode in the endless dungeons of Angband, the Hells of Iron, for in the War of the Powers the Valar, in their haste to overthrow him in his great stronghold of Utumno, did not wholly destroy Angband nor search out all its deep places. Beneath Ered Engrin he made a great tunnel, which issued south of the mountains; and there he made a mighty gate. But above this gate, and behind it even to the mountains, he piled the thunderous towers of Thangorodrim, that were made of the ash and slag of his subterranean furnaces, and the vast refuse of his tunnellings. They were black and desolate and exceedingly lofty; and smoke issued from their tops, dark and foul upon the northern sky. Before the gates of Angband filth and desolation spread southward for many miles over the wide plain of Ard-galen; but after the coming of the Sun rich grass arose there, and while Angband was besieged and its gates shut there were green things even among the pits and broken rocks before the doors of hell.
“Of Beleriand and its Realms” - The Silmarillion - J.R.R. Tolkien