In June of 2018, The Wild Boars, a teenage Thai football team, went into the labyrinthine caves of Tham Luang on a team-building exercise, as they had apparently done several times before. It was supposed to take an hour or so, as it usually did. This time, though, it rained, flooding the caves, and the rain didn’t stop until all 12 players and their coach were royally stranded. The next 18 days kept the world on tenterhooks, and the astonishing outcome fueled a whole host of documentaries plus a Ron Howard Netflix drama, Thirteen Lives, starring Viggo Mortensen, Colin Farrell and Joel Edgerton.

In an unusually inoffensive stroke of Hollywood hero-casting, Edgerton played Richard “Harry” Harris, the Australian anesthetist and cave diver who risked his professional reputation and possibly his lifelong freedom by suggesting a strategy of putting all the boys into a chemically induced kind of coma so they could be carried out by experienced divers. They all made it out alive, as we know, but Deeper takes us five years forward into the real Harris’s story, making this a sequel of sorts, albeit in a most unexpected way.

In his own words, Harris insists, “I’m not a brave man,” and Peedom seems surprisingly happy to take that at face value. But, as we are soon to find out, bravery isn’t really the issue here. Harris is a driven man, and even he is at a loss to explain where his obsession with deep-diving came from (and he does call it an obsession). When we first meet him in Deeper, this drive has taken him, eight times already, to The Pearse Resurgence in New Zealand, which he describes as “potentially” the deepest-dived cave in the world. To this end, he has gathered a highly respected team of divers called The Wet Mules.

That “potentially” is important, since Harris and his crew have designs on going further down than any previous divers, and, calling back to the events of 2018, Harris has an equally experimental plan in mind, one that can go either way. (“There are not a lot of cave-diving accidents,” says Harris’ long-suffering wife Fiona. “You die or you don’t.”) To counteract the shakes and accompanying anxiety that occur under heavy pressure, this particular idea involves making a hydrogen cylinder part of every deep diver’s already crowded backpack, a move so untested that the director films Harris’s first attempts to fiddle about with it from a very safe distance. “If you don’t handle it correctly,” muses Harris, “it can cause problems.” In layman’s terms: It explodes.

But will it? Peedom treats the threat responsibly, never teasing us with the possibility of it all going horribly wrong (the talking heads and present-day voiceovers seem custom-built to be reassuring). Neither are the visuals as forbiddingly claustrophobic as you might expect, given the spectacular vertiginous thrills of her immersive 2017 film Mountain – this is not an underwater Free Solo. And when the whole project seems to go south, you might think you’re in for another switcheroo like 2015’s Sherpa, in which the film’s focus changes quite radically following a disaster on Everest.

But, leaving aside the oddly unaddressed question of who’s paying for this, what is it all for? Instead of hyping up the omnipresence of death, which is baked into the premise, Peedom opens up a debate on the ethics of the experiment itself. When does obsession become a death wish? When do, or should, we intervene? This is not a pejorative question; as Harris’s wife says herself, and as the whole world saw in 2018, sometimes a deep-diving anesthetist can come in handy.

But as she also observes, although her husband’s friends are very intelligent people, they don’t half like to play with fire. And, in the end, that’s what the film will leave you thinking about: the human cost of progress. “They should know when enough is enough,” says Fiona. “But will they?”

Title: Deeper
Director: Jennifer Peedom
Sales agent: Dogwoof
Running time: 1 hr 27 mins