Brandon Hendrickson had already been submerged face down in the pool for more than four minutes, motionless under the dappled Grand Cayman sky, when the struggle phase began. That’s the term of art among free divers and competitive breath-holders for the point at which the human body, sensing an alarming rise in internal carbon-dioxide levels, tries to forcibly override the will. For most people, the respiratory muscles begin contracting within a minute or two, and these involuntary breathing movements, or I.B.M.s, trigger an immediate intake of air. But for élite free divers such as Hendrickson, who grew up spearfishing in Florida but now runs a tree-care service in landlocked Olathe, Kansas, the struggle phase is just the beginning. Some pros endure more than a hundred I.B.M.s before, finally, they yield.
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