Realm of Ossiriand
Thangorodrim (elvish for ‘oppression mountain group’) was a group of three volcanic mountains in the Iron Mountains.
The highest peaks of Middle-earth, they were raised by Morgoth, who delved his fortress of Angband beneath them, and far back into the Iron Mountains.
Thangorodrim was said to have been the piles of slag from Morgoth’s furnaces and rubble from the delving of Angband, but at the same time they were solid enough to form sheer precipices;
The tops of Thangorodrim perpetually smoke, and sometimes spewe forth lava. The three peaks of Thangorodrim function as furnaces for Morgoth’s great smithies deep in Angband.
At the base of the south face of the middle peak is the Great Gate of Angband, a deep canyon leading into the mountain, lined with towers and forts. There are also a number of secret gates scattered around the sides of the mountain group, from which Morgoth’s hosts could issue forth and surprise their foes. The position and size of Thangorodrim are 35,000 ft high and it lays a distance of 150-200 miles north of Menegroth but the plateau of Dorthonion forces a long detour which adds an extra 200, 250 miles to the distance.