How big was Ancalagon?

[narfforc]

I do not think we can compare The Towers of Thangorodrim to any mountain or range of which we know. What we do know is that Morgoth made them from the vast (newer) delvings of Angband. These new delvings occured when Morgoth returned from his enforced stay in the house of his brother Mandos. In the Silmarillion it states:- 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 refuge of his tunnellings. They were black and desolute and exceedingly lofty; and smoke issued from their tops, dark and foul upon the northern sky. So, these lofty peaks seem less like great mountains, and more like huge chimneys for the darkness of industrialisation, something which pops up time and time again in the Life and Works of Tolkien. The towers of Thangorodrim are also described in The Silmarillion as reeking. It is possible that the strength of these towers have been somewhat overstated, Slag, ash and rock can be packed quite tightly, however, if reeking black smoke is issuing from fissures high on its peaks, it give thought to how that smoke gets there. I believe that it travelled up through the towers, therefore making the towers of Thangorodrim full of tunnels and cracks, and maybe very unstable.