In May this year in Honduras, Sayuri Kinoshita held her breath, dived into the ocean — and disappeared. Swimming in the undulating manner of a dolphin, she had vanished down into the darkness. Kinoshita was taking part in the Caribbean Cup free-diving competition. She was attempting to beat her own world record for the depth reached, in a category of diving where you are not allowed to wear fins to help you swim, nor to pull yourself down with the aid of a rope.

This category — constant weight no fins (CNF) — is considered one of the most challenging of the disciplines of free diving because not only must you hold your breath while descending, but you must also put in immense physical effort to propel yourself downward. Kinoshita, born in 1988 in Nagasaki (she now lives in Okinawa) didn't break her record at the Caribbean Cup, but then no one has yet broken her record of 72 meters that she set in 2016.

Kinoshita became interested in this extreme sport after reading about the success of another Japanese free diver, Hanako Hirose, and says the sport is growing in popularity in Japan. It's a modern version of the traditional ama divers of Japan, mostly women, who dive to collect pearls and seafood. These women swim in the same undulating way — like a "mermaid," says Miku Higuchi, who at age 20 is perhaps the youngest of the traditional divers in Mie Prefecture.