Apocalyptic Books Where the Apocalypse Is Never Explained

My favourite genre of apocalypse novel is one where the nature of what ends civilization as we know it is never quite explained. Maybe it's hinted at, or maybe you know WHAT happened but not HOW or WHY it happened. And that's usually because that's not the point. An unknowable catastrophe, one that can't be predicted or understood, is far more frightening than something as simple as asteroids or zombies.