Board Thread:Suggestions forum/@comment-27200931-20170426224854/@comment-27200931-20170428023315

Gandalfthegreatestwizard- EpicMithrandir wrote: High King Ithilion wrote: Utumno wasn't in use during the War of Wrath, so it wasn't destroyed...

Ithilion, Discussions Moderator (Auta i lómë) 02:22, April 28, 2017 (UTC) It was. Tolkien Gateway says it, and no doubt The Sil would agree if I had a copy handy. "The Valar attacked Udûn in full force, and destroyed it utterly, carrying its master back to Valinor as their prisoner."

This is when Tolkien Gateway says that Utumno was utterly destroyed. It refers to that quote I posted earlier, which also states that an unknown quantity of caverns and vaults remained.