In the South Korean province of Jeju lives a group of women who may, at first, sound as if they’re mythical creatures. The haenyeo (“sea women”) dive and swim in the depths of the ocean without any breathing gear, harvesting seafood like conch, urchin and octopus, which they sell to support their families. Often, haenyeo are the main earners in their households. They’ve done this for centuries.

That is remarkable enough on its own. Free diving — without breathing apparatus other than your own lungs — is incredibly physically demanding and dangerous. Jellyfish and sharks lurk, and bad weather can pose a hazard, too. Some of the haenyeo dive for two full minutes without coming up for air, all while gathering marine life.

But what’s more extraordinary — and what’s explored in “The Last of the Sea Women” (on Apple TV+), directed by Sue Kim — is this astonishing fact: Most of the haenyeo are in their 60s, 70s and 80s. For them, it’s both a point of pride that they’re still doing the work — “men can’t handle this job,” Soon Deok Jang, 72, notes with a grin — and a grave concern.

That is because they have been at this a long time. Yet in recent decades, their numbers have dwindled from tens of thousands to about 4,000. Few young women join the community anymore, and the divers worry that their way of life is disappearing. “I feel like the haenyeo culture is melting away,” one says, referring not only their work, but also their camaraderie and empowerment.

Kim takes a lightly ethnographic approach, melding an observational eye with conversations with the women. She follows the haenyeo into the sea, hangs out with them as they wait at the crack of dawn for transportation to the water and attends meetings where they discuss how to preserve haenyeo culture.

And, as the film shows, there are a few encouraging signs. There’s a school for new haenyeo, though the grueling demands of yearlong training and the work itself mean only about 5% of graduates continue on. The film also focuses on Sohee Jin and Jeongmin Woo, who are in their 30s and the youngest by far among the haenyeo. They initially connected over that fact, and now they post videos to YouTube and Instagram to draw attention to their work.

Still, there are plenty of threats, and they keep coming. Perhaps the smartest thing that “The Last of the Sea Women” explores is the importance of the haenyeo in detecting changes in the underwater ecosystem. When you dive deeply and observe marine life carefully for hours every day, you notice subtle and not-so-subtle changes. At the same time, a lot of existing marine life seeks deeper, cooler waters, which makes harvesting harder. And there’s a limit to how deep a human body can dive

A substantial section of the film deals with Japan’s release of treated radioactive wastewater from the ruined Fukushima nuclear plant into the ocean, which began last year and could last decades. Though the International Atomic Energy Agency and other experts have deemed the plan safe, it has caused tension with China and South Korea, and participants in the film argue it will slowly harm the Jeju waters.

“The Last of the Sea Women” makes the compelling case that losing a centuries-old way of life — whether to a tradition dying out or to contaminated waters — is a tragedy for everyone. A connection to history disappears, as well as a community deeply invested in marine health. For the haenyeo, being “guardians of the sea” is a calling for women across generations that “you pour your heart into,” as one diver puts it.

What’s most lovely about “The Last of the Sea Women” is the humanism of its portrait of the haenyeo, balancing humor and friendship and creativity with serious concerns and a sense of elegy. Near the end, one haenyeo is reflecting on who she sees herself becoming and what she hopes the future will look like. “Just an old woman and the sea,” she muses. “Forever.”