There is a long history of breath-hold diving cultures in East Asia, with references in Japanese chronicles as early as the third century BC. Given evidence of genetic adaptations for phenotypes associated with enhanced diving capacity within such populations1, it is likely they hold the most prodigious human diving abilities — abilities that may be akin to semi-aquatic mammals, and even some marine mammals. Yet, a dearth of fine-scale information exists on the combined natural diving behaviour and physiological responses within these diving populations. One such extraordinary population is the all-female Haenyeo. Here, we assess the fine-scale diving behaviours and physiological responses of these women during natural harvest diving. Our results show that Haenyeo divers demonstrate the highest proportions of time underwater of any humans, also exceeding those of semi-aquatic mammals and being comparable with some marine mammals. Additionally, they do not exhibit an overt cardiovascular depression, or ‘dive response’, classically associated with consummate diving mammals.