Should you find yourself falling behind on your summer fitness goals, you might find late-breaking inspiration in The Deepest Breath, which makes running a marathon look like a mosey through the park. Laura McGann’s stunning documentary is a love story embedded in the world of free diving, the deadly extreme sport whose practitioners plunge toward the ocean floor, often wearing nothing but a wetsuit and nose plug. (Goggles are out of the question since the air pressure would cause blood vessels in the eyes to burst.) The pursuit is no less psychologically than it is physically challenging. Divers must retain their focus yet transport themselves to a disconnected mental state in order to withstand the pressure of the depth, which can burst eardrums, crush lungs and bring on blackouts. Natalia Molchanova, the Russian champion free diver who broke world records until the waters of Ibiza swallowed her in 2015, created a method of completely emptying her mind and surrendering to the silence. Molchanova was the idol and inspiration for Alessia Zecchini, one of The Deepest Breath’s two fiery protagonists. The Italian supernova began breaking records while training as a girl among men at a pool in Rome, but had to wait until she was 18 to compete. Lithe, wavy-haired and hellbent on greatness, she is this summer’s other little mermaid. “Alessia was a crazy head,” a fellow female diver recalls in the film. The Deepest Breath is sparing with its talking-head interviews, mostly availing itself of footage that exists from training sessions and competitions in years past. McGann only began making The Deepest Breath after reading a newspaper article about the tragedy that lies at the heart of the film, but the free diving community is a band of documenters who travel with their own state-of-the-art gear, it turns out. The images that she weaves into the film incorporate seamlessly with her own beautiful shoots. Apart from Alessia, there are two key interview subjects: her father, Enzo, and the father of Stephen Keenan, the Irish safety diver who only appears in older footage, and who people talk about in the past tense. This is not a story with a surprise ending, which makes the knife-edge tension that powers the film all the more impressive. Rounding out the pervasive sense of fear and ecstasy is a mesmerizing, sometimes mind-altering, depiction of the ocean’s depths. When one beholds Zecchini’s figure undulating to the sound of nothing, it’s all too clear that thrill-seeking is only part of the story. There was something of the lovable rapscallion about Keenan, who from a young age wanted to find something bigger than his Dublin neighborhood. His father expresses regret for leaving the family home when Keenan was still in school. Stephen was close to his mother, who died of cancer when he was young. Inspired in part by his childhood love of David Attenborough, Keenan lit out for adventure. While his friends were starting to settle down, Keenan traveled across Congo, Sudan, Ethiopia. He fell in with a group of free divers in Dahab, Egypt, the coastal town on the Sinai peninsula that became his home base and where he eventually opened his own diving school. A skilled diver, he traveled around the world as a safety diver, a key role at competitions where aspirants often pass out underwater or emerge with their eyes rolled to the backs of their heads, in need of resuscitation. In addition to their love for the sport, he and Zecchini had in common a wild streak, and they clicked. He was the trainer who was able to get through to the headstrong Italian, and coax her toward a series of world records. For many years, Molchanova was the only female diver to make it through the arch in Dahab, a dark and treacherous underwater corridor. Emboldened by a handful of recent successes, Zecchini set her sights on Dahab. “Once you start touching world records, that’s when things get worrying,” Alessia’s father, Enzo, says in an interview, his eyes looking rheumy. The film’s final act chronicles the duo’s efforts in the days before what would be a fatal incident. Shortly before their final dive, Keenan sent Zecchini a video message. “Live for today for you never know what’s coming down the line,” he told her. The pair’s passion is undeniable – they are both drunk on the same underwater dreams for Zecchini, and their above-ground attraction to each other. Keenan’s life was cut too short, but nobody can say he didn’t find what he was looking for. The Deepest Breath is out in cinemas on 14 July and on Netflix on 19 July