The film opens in Greece in the 1960s, where the boys Jacques and Enzo grow up by the water and dare each other into it. Their rivalry is gentle at first - a coin dropped to the seabed, a challenge to fetch it - but the sea is not gentle. Jacques watches his father, a hard-hat diver harvesting the seabed by air hose and helmet, drown when his line snags on the rocks. That wound never closes. Besson based the story, loosely, on real champions: Jacques Mayol, who lived from 1927 to 2001, and Enzo Maiorca, renamed 'Enzo Molinari' for the screen and played with brooding warmth by a young Jean Reno. The childhood scenes were shot in the stark, sun-bleached light of the Cyclades, and the islands become almost a third main character.

Much of the film was shot on Amorgos, the easternmost of the Cyclades, and the images that lodged in viewers' memories belong to it: the little chapel of Agia Anna perched above impossibly blue water, and the monastery of Panagia Hozoviotissa, a white slab of a building wedged dramatically into a sheer cliff face high above the sea. Decades later, fans still make the trip - up the goat path to the chapel, along the cliff to the monastery - retracing shots from a movie about men chasing something just out of reach. Scenes were also filmed at Manganari, on the southern coast of Ios, the same island that claims Homer's grave. The Aegean did not need much help looking otherworldly.

What makes The Big Blue strange and enduring is that it is not really a sports film. It is about a man who cannot stay on the surface. Mayol, in life and in the film, was fascinated by dolphins and by the limits of the human body underwater; he was genuinely recorded as having a heartbeat that slowed from 60 down to 27 beats per minute when he dove, his body shifting toward something almost aquatic. The movie pushes this into myth. Besson's Jacques is drawn downward by a longing his girlfriend Johana, played by Rosanna Arquette, can sense but never share. 'I'm alive,' she pleads near the end. 'Whatever is down there is not.' He goes anyway. The depths win.

The real story diverged sharply from the screen. Mayol and Maiorca were both genuine champions and contemporaries, and both set no-limits depth records below 100 meters. But they did not compete head to head as bitter rivals, neither ever reached the 400 feet the film depicts, and neither died in the water. Maiorca went on to retire into Italian politics, even serving in the Senate; for years he resisted showings of the film in Italy, feeling it caricatured him, and only relented after Mayol's death. Mayol himself helped write the screenplay - and the ending's melancholy proved tragically prophetic. After a long depression, he took his own life in 2001, more than a decade after the film. President Jacques Chirac called him a symbol of the 'Big Blue' generation.

By any normal measure the film should have been a curiosity. American critics called its plot pointless and its pace glacial; one warned viewers they would 'want to come up for air' within minutes. Recut with a tidy happy ending and a new score, it flopped in North America. But in France it became a phenomenon, selling more than nine million tickets and running in theaters for a full year - the most successful French film of the 1980s, scored by Besson's longtime collaborator Éric Serra with music as slow and deep as the dives. It remains one of cinema's great cult objects: a movie that taught a generation to look at the sea differently, and sent them climbing to a chapel on Amorgos to see the blue for themselves.