The Ered Engrin, or Iron Mountains was the greatest mountain range in Middle-earth and formed a great arc from east to west. They were built by Melkor with at formation of Arda. His original fortress of Utumno and his lesser fortress of Angband were both delved deep below the Iron Mountains.

After the War of Wrath at the end of the First Age, the world was changed greatly, and the Iron Mountains were greatly diminished, either by reduced in height, or taken down completely below the sea.