During practice dives, David says he could feel carbon dioxide building up in his body as the clock ticked closer to 16 minutes. "That's when it's the fight," he says. "I would say the last six minutes is when—even though I have to remain perfectly still—I'm actually fighting the hardest. If you see me start to do these convulsions, then that's the real hard part."
David says it's so difficult because the body has a natural urge to breathe. "It's overwhelming," he says. "You're fighting really hard to override what your brain is telling you your body needs to do."
When the pain starts to build, David says he tries to remove himself from the moment. "I imagine getting sucked into the abyss of the ocean almost," he says. "I don't have any thoughts. I empty everything out and become perfectly still."
Sinking into another level of consciousness allows David to slow his heart rate and change the way he experiences time. "When the heart rate slows down, everything slows down. Time changes during that," he says. "It doesn't feel like 16 minutes, which is a good length of time. It feels more like one moment compressed."