12 Huge Movie Mysteries That Are Still Unsolved

5. What Was The Trigger? - Children Of Men

Children Of Men's grim premise sees humanity facing the very real possibility of their own extinction after the world's population is rendered infertile. Well, apart from one pregnant mother-to-be, who becomes the hope of the entire species.

The mystery here is what exactly happens to trigger the mass-infertility event. There are some hints, dropping like crumbs throughout the film, but they're no more than theories of what happened, rather than actual answers. Was it divine intervention as punishment for humanity's sins? Was it down to environmental shifts? Alfonso Cuaron refuses to answer beyond those hints, probably because of his disdain for exposition.

Instead, he keeps the full picture shrouded from the audience (and there's no more hint in the source material), so it seems unlikely there'll ever be a definitive answer.