I think No.6 reached the Underworld long before he reached the cavern in 'Fall Out! In fact the way that steel door opens, it's almost as though No.6 has passed through one of the nine doors of hell!
"What are you doing down there?"
"I fell over."
"Why don't you stand up like a man?"
"Who, who are you?"
"I could be a friend."
Of course No.6 isn't in Hades, the Underworld, but he is underground. What's more he's been here before, the Manager's office of the Labour exchange!
It's very surreal looking, almost like something from a 1920's film. The way No.6 arrives in the Manager's office is surreal as well. No.6 disappears through the floor of the Council Chamber, then struggles along the tunnel by hanging from the leather straps, suggests that the office in which No.6 ultimately arrives is below the Council Chamber in the Town Hall, which of course it isn't, because it's beneath the Labour Exchange. And after undergoing the "truth test" No.6 eventually emerges from the Labour Exchange!
Be seeing you