Second movie staring Alexander Skarsgård and Andrea Riseborough, the first being Disconnect (2012), although they did not share any screen time in Disconnect.

The movie contains no credits at the start, just the title of the movie.

The little girl says 'They're here' - an homage to Carol Anne saying the same thing in Poltergeist.

"Rule #2 : Never lose control" is mentioned a couple times in the first half of the movie. It is kind of odd so high placed on their survival list. This is perhaps the only clue to see the end twist coming. But you also see Ray spending a long time trying to calm Zoe who is scared by telling her a story, and then when the first breather almost opens the vault because of Olive's voice, Ray is literally shaking beside Claire on top of the ladder as she tells him "he won't get in" trying to soothe him. So the build up is there, it took me two looks at it to really be aware of the clues!