That nagging, need-to-breathe feeling you get holding your breath is triggered not by oxygen deprivation but by buildup of CO2. Comfort with this buildup is what separates good freedivers from great ones, or good ones from guys like me. Freedivers condition their bodies to tolerate high levels of CO2 using timed breath-hold exercises called static tables. Essentially, it’s interval training. Breathe two minutes, take four huge breaths, hold breath for two minutes; breathe one and a half minutes, take four huge breaths, hold for two and a half minutes, and so on.
