Board Thread:Suggestions forum/@comment-69.157.69.164-20150501192325/@comment-26298013-20150510014851

I said what I said because, in the Silmarillion, Tolkien states that the very lowest pits of Utumno were laid open to the sky. Oh, and that all of Utumno was entirely and completely destroyed.

It DOES, however, mention that Angband was not, at first, entirely destroyed. I don't know about what happened to it after it became Thangorodrim.