That’s part of the plot of the movie

Edit: it sucks that I can’t say the movie without ruining its surprise, what a useless spoiler tag 😭

For anyone who doesn’t want to ruin the surprise: it’s one of these movies: 10 cloverfield lane, cloverfield, and descent

There, now you won’t know which one exactly (if you haven’t seen all three) but you’ll get a ballpark sense of which movie to watch