Armed only with a harmonium, Juliette swam through a world lost underwater in the fourth episode of “Silo” Season 2 — and that meant star Rebecca Ferguson spent a lot of time in the water tanks filming that particular episode.

“I dive a lot, when I can,” Ferguson says in a behind-the-scenes video from the day of the shoot. But that doesn’t mean that the day will be made easier. In fact, in many ways it was made even more difficult because Ferguson is playing someone who doesn’t know how to swim; after a lifetime in a bunker, Juliette has never spent this much time underwater.

“We built a set about 15 feet underwater, and we’re having some fun today,” episode director Michael Dinner said. “We built this tank from scratch. It’s like the second biggest water tank in the U.K., and we’ve just made it for this production,...