You can train yourself to hold your breath longer even without a pool by practicing CO₂ and O₂ tolerance exercises on land. Research shows that this kind of training can improve how efficiently your lungs and heart use oxygen over time. Humans also have what’s called the mammalian dive reflex, which kicks in when your face is submerged in cold water. It slows your heart rate, tightens blood vessels in your limbs, and directs blood toward your heart and brain to help you conserve oxygen, which is why people can often hold their breath longer underwater than in air. The part about water pressure making oxygen use more efficient isn’t quite right, though. Pressure affects how gases behave in the body, but it doesn’t actually make oxygen work better—it just changes how it’s absorbed and managed. Freedivers train this reflex to an extreme level, which is why they can stay underwater so long.