“Alessia, how do you feel about death?” asks an off-screen interviewer in the opening scene of The Deepest Breath. “Honestly,” answers Alessia Zecchini, a 31-year-old professional free diver, “I’ve never thought about it.” The documentary immediately sets out to demonstrate the profound contradition between this statement and the realities of free-diving, an extreme sport in which a diver descends hundreds of feet into the ocean with just a single breath of air. The scenes where we watch Zecchini’s dives are serene—she describes the depths as “the last quiet place on earth”—but disquieting. We see her experience all the potential dangers of a dive: at a certain depth, for instance, the ocean’s pressure begins to drag a body down into “free fall”; and on the ascent, divers can temporarily black out when the pressure change causes the brain to shut down. Amid such perils, The Deepest Breath finds its rhythm in the story of Zecchini and her safety instructor, the Irish free-diver Stephen Keenan. —Paulina Prosnitz