Dwarfed by the sperm whales swimming overhead, world champion freediver Guillaume Néry appears mesmerised by the graceful movements of the majestic creatures and the interplay of light and motion beneath the waves. The man capturing this dramatic image is Franck Seguin, one of the world's leading underwater photographers and a great admirer of freedivers, who descend to great depths while holding their breath rather than relying on breathing apparatus. "They reach places that most of us can't go," he says. "What they do is more than an athletic achievement – it's a search to find harmony with nature."

The images are part of One Breath Around the World – a collaborative project that saw Franck and Guillaume freediving through ancient rock formations, exploring the Yucatán's famous cenotes, climbing underwater trees and even meditating on the ocean floor.

The pair met through the freediving community and became close friends after Guillaume retired. Inspired by tales of epic journeys crossing continents, they decided to embark on a similar odyssey – underwater. Combing through spectacular dive sites, they finally settled on six locations – the Philippines, Mexico, Mauritius, French Polynesia, Japan and Finland. "We decided to explore the freediving world through oceans," says Franck, a Paris-based Canon Ambassador who heads the photo production department at L'?quipe newspaper.

Being unencumbered by heavy scuba gear lent a freedom to Franck's photography, but also left him precious little time to get the shot. While Guillaume can hold his breath for a staggering eight minutes, for safety reasons none of the team stayed underwater for more than two minutes at a time – which is about the limit of Franck's lung capacity.

"Swimming in the ocean far from the coast, it can become dangerous at any time," says Franck. "The most important thing is to never exceed your limits. It's not important to be a freediver with a very high skill level – you need to think of your picture before entering the water, be in the right place and shoot quickly."

Carefully choreographed above the water, the images could then be captured swiftly on each two-minute dive. The pair would then return to the surface for air, before diving again – sometimes as many as 10 to 15 times.

"Underwater, everything is more complicated," he says. "The focus, being able to use the viewfinder, triggering the shutter release, and the light settings – even the fact that you're constantly moving. Shooting with a fast lens means the technical difficulties are halved, and that's huge in such tricky working conditions."

Franck says his "greatest memories" came at the end of their trip in Mauritius when, after days of searching the waters with a hydrophone – an acoustic probe – to hear whale calls under the surface, they found a group of 15 sperm whales.

Sperm whales made natural subjects for this project – record-breaking freedivers themselves, they can dive to depths of 3,000 metres for up to 90 minutes. To discover such a large pod, including mothers and calves, sleeping vertically, exceeded all expectations. And diving while holding their breath meant Guillaume, Franck and Julie weren't releasing air bubbles and were quieter in the water, allowing them to swim silently around the sleeping giants.

"Guillaume dove towards them vertically, with his arms outstretched to stabilise himself," recalls Franck. "I was following him, freediving with my fin. Everything balanced itself. All my settings were done on the surface. I was confident, the light was magical. When I looked at the pictures, I realised it was a gift from a nature."

The hour and a half spent with the pod is something that will stay with Franck forever. "Sperm whales are so special," he says. "During the dive, Guillaume and I tried to be accepted by the group, and we both felt they allowed us to be there and [allowed] me to take pictures. It was a magical encounter in the Indian Ocean."