The Iron Mountains or Ered Engrin were an immense mountain range in the far north of Middle-earth, stretching almost from one end of the continent to the other, though falling short of the sea on either side.
Of old the Iron Mountains came close to touching the Blue Mountains (Ered Luin) of the West and the Red Mountains (Orocarni) of the East, but in the wars between the Valar and Melkor the mountain range was distorted. Melkor himself created the mountains as a fence for his citadel of Utumno.
Melkor's great fortresses of Angband and Utumno were built in the mountains. North of the range lay the regions of ever-lasting cold.
After the War of Wrath the Iron Mountains were broken and disappeared, at least the part of their length that lay directly north of Beleriand.