My heart begins pounding the theme from Jaws, triple tempo, as the shipwreck inches into view. Given the task I face—kicking down 55 feet on a single breath—such anxiety is a perfectly normal response, although precisely the wrong one for a free diver. Fear releases adrenaline, which jacks up the heart rate, constricts blood vessels, and causes rapid, shallow breathing. But if I can relax, an entirely different experience awaits: The exhilaration of probing the deep blue unencumbered by air tanks.