Thangorodrim

Thangorodrim is a group of three volcanic peaks in the north of Middle-earth in the First Age, raised by Morgoth out of the slag and rubble thrown up by the re-delving of his fortress of Angband. The Silmarillion counts them the tallest mountains of Middle-earth, though no measure of them survives. For the most part they do little but stand, throwing out fire and molten rock when Morgoth is in a rage or is loosing an assault. The Great Gate of Angband opens at the foot of the middle peak on its southern side, a deep canyon lined with towers and forts, and secret gates are cut here and there about the mountains so that his armies can come out where they are not looked for. At Manwë's bidding the Great Eagles keep their eyries on the three summits and watch what is done below.