Dwarfbane Cavern

This cavern has vast unending, looping, and otherwise labyrinthine tunnels that are both rich in minerals to mine and rich in monsters; so much in fact that even the stalwart and hearty dwarves could not conquer it.

One possible explanation floating around for the closure of the Dwarven Gates is that the cavern houses something so terrible that is not only killed the dwarves who explored it, but that it found its way through the tunnels into the ancient dwarven city itself and killed everything behind the gates such that there's simply no one left alive who can open them.