Video narration
What if you could swim underwater without ever needing to surface? At the pool, you know that familiar moment: the urge to breathe eventually pulls you back up. But it isn't about holding your breath longer. It's about getting oxygen from water. Human lungs take oxygen from air; they can't pull it from water. Water contains oxygen in dissolved form, but you can't inhale it the way you inhale air. Gills solve that problem: as water flows over them, they extract dissolved oxygen and pass it into the body. That would take more than adding gills. A human body would need major changes to move water across them and deliver oxygen throughout the body. Exactly how that human body would work is speculative. Could humans breathe underwater? Only with radically different bodies: gills to draw oxygen from water, instead of lungs built for air.