Hanli Prinsloo, a South African freediving coach, divides the progress of a dive into four stages. First is a subtle “awareness phase,” where the urge to breathe begins to assert itself in your consciousness. If you push past that, you’ll start to feel involuntary contractions in your diaphragm—a response to the buildup of carbon dioxide in your blood rather than the lack of oxygen. This you can safely (but temporarily) ignore, if you’re willing to suffer. Then comes the welcome rush of fresh blood from the spleen, offering a psychological boost and allowing you to extend your dive. Finally,
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