Sharks and confined spaces are both big fears for much of the population - so combine the two and you've got what looks like a genuinely terrifying film.
47 Metres Down is the latest film in the shark attack horror canon that started with Spielberg's Jaws back in 1975 and continued most recently with last year's so-so The Shallows.
47 Metres Down at least looks more involving than that film because the sharks in question don't display an unconvincing preoccupation with Blake Lively.
Judging by the trailer, released this week, the plot seems paper-thin but action-packed: Mandy Moore and Claire Holt play a couple of holidaying girlfriends who decide to take the plunge and go cage diving with sharks.