As night fell, Laurent Ballesta headed for the abyssal drop-off at the edge of the reef. He preferred the thought of spending the hours of darkness in the black waters of the deep ocean than in Fakarava Atoll’s shallow southern channel, where hundreds of frenzied grey reef sharks gorged on camouflage groupers.

When Ballesta had started his 24-hour solo dive earlier that day, forward-rolling into French Polynesia’s warm waters beneath a blazing sun, he hadn’t anticipated leaving the 30-metre-deep coral-clad channel. He was there to observe the aggregation of some 18,000 camouflage groupers for a full 24 hours, which meant staying with them in the pass throughout the night.

Such a lengthy period of time with the fish represented a significant opportunity for behavioural observation. The groupers had gathered in the pass to mate, as they do every year, but beyond the keys facts regarding their two- week aggregation period and the brevity of the spawning itself – just 30 minutes – very little was known about the event. Marine biologists on standard open-circuit dives typically have about an hour in the water at a time. That’s not much time for observation and offers very little chance of witnessing the spawning event directly. Ballesta had joined the groupers for a full day and night to watch for spawning cues and clues, to learn as much as he could about their behaviour and, just maybe, see something rare and remarkable.

As the sun disappeared over the horizon, Fakarava morphed into an altogether different place from that which Ballesta had encountered during the day. The sharks that had been asleep in the pass during the hours of daylight, gently swimming against the mellow current between lagoon and open ocean, had awoken to hunt. The frenetic activity of predators and prey quickly became unnerving. Ballesta needed space. Moving on the same current that would sweep grouper eggs and semen out to the open ocean in the coming days, he moved away from the chaos – bumped by sharks as he went – and out of the pass. Ballesta and his team knew, of course, that the sharks would be more active at night; it was the scale and manner of hunting activity that came as a surprise. As unsettling as the shark activity was, Ballesta, as a filmmaker and biologist, found it fascinating – he hadn’t seen sharks behave in such a way before. It seemed to him as if they were hunting in packs. However, 2014’s Gombessa II mission – backed by long-term supporter Blancpain through its Ocean Commitment programme, an initiative that has so far funded five Gombessa missions and helped protect a staggering 1.2% of the planet’s ocean – wasn’t focussed on Fakarava’s sharks.

“We were not there for the sharks,” says Ballesta. “For my entire career to that point, I had always avoided shark stories. There are hundreds of shark films and, most of the time, they are the same. Sharks are fascinating, of course, but my focus is – and always has been – on what’s new. What can I bring in terms of science, diving challenges or wildlife images and filmmaking that hasn’t been done before? When I heard about this huge aggregation of camouflage groupers – everybody was saying thousands of them gathered at Fakarava for one or two weeks every year to mate, but nobody had really seen the spawning – it seemed a real wildlife challenge.”

Fakarava’s grouper aggregation also represented a compelling enough reason to take on another challenge he had dreamt about for a long time: the 24-hour dive. Fakarava’s southern channel – calm, shallow and full of biological mystery – was the perfect place for him to test himself and spend a day underwater, a feat of human endurance in the name of behavioural science and wildlife photography.

“Time at depth with the groupers was a precious thing, and would offer me an opportunity to watch the aggregation like no one had before,” says Ballesta. “By doing a 24-hour dive, I would have the time to observe the fish more deeply. Our hope was that by studying grouper behaviour over a longer period of time we might be able to better understand when they would spawn.”

The completion of 24 hours underwater made Gombessa II a huge success regarding the dive objective, though Ballesta didn’t spend as much time observing the grouper as he had anticipated because of the shark activity and his subsequent escape to the sanctuary of the open ocean. “According to our GPS data,” says Ballesta, “I swam 21 kilometres that first night, simply because I didn’t want to stay in the middle of the channel. When I stayed still for just a few minutes suddenly 10, 20, 50, 100 sharks or more surrounded me. So I spent my time on the move. By swimming I kept the number of sharks around me to about 20. I just was not confident enough to spend twelve hours alone with those sharks.”