Korean actress Song Ji-hyo is swapping the film set for the depths of the seas. She is the center of Deep Dive Korea: Song Ji-Hyo’s Haenyeo Adventure, a doc series from BBC Studios and Jtbc.

Across three episodes, Song will free dive with the haenyeo underwater sea fishers, whose tradition has been passed down from generation to generation over 400 years. They risk their lives by plunging up to 20 meters to the seafloor without oxygen masks or advanced equipment to gather shellfish and many of the woman are in their 80s.

Song’s mother was a former swimming athlete, while her aunt was a haenyeo, who usually begin their training at age eight. The actress in now in her 40s and will have to cram in three decades’ of training into just a month.

The doc will see her submerge herself in haenyeo techniques and culture, while taking viewers under...