I have been through a lot of training over the years. After a few beers, people sometimes ask about the toughest training I have ever seen. After you read this story, I think you will agree that the Combat Diver Qualification Course (CDQC) at the Special Forces Underwater Operations School in Key West, Florida, takes the prize.

I went through CDQC before it was a seven-week marathon — my hat is off to those guys who took the course after I did. The drills have since changed, but the objective remains the same. If you have any trace of claustrophobia or fear of drowning, the clever instructors will find it. They will push that button again and again until you forever ignore it or quit. The brain has special circuits to warn you that you are dying and cause panic. Key West teaches you to turn them off.

There is no try. Every man in my class who failed to graduate quit in pool week. Whereas in Ranger School, there were hours and miles between meals where you thought about how powerfully hunger sucked.

Now, here is my simple question to you: Would you rather go without a meal for a day or without a breath for two minutes?

The first week, they issued full open circuit gear, complete with twin 80-cubic-foot tanks. While we got plenty of experience donning and wearing these primitive behemoths, we did not turn on the air and use regulators until week two.

My least favorite exercise was bobbing, where you jumped up from the bottom of the deep end with your fins on your hands, got your head above the surface, took a breath, and went back down. Repeat until told to stop. The purpose of this drill was explained several times, but the story was never the same twice and never made any sense. I did this correctly only once, on test day.

There was another drill called crossovers. Half the class would line up on each side of the long axis of the Olympic-sized pool. At that point, there were about 25 guys still in the class. They would push us into the deep end so everyone was crowded and with Twin 80s, BCD vests, mask and fins, and weight belts, we clung to the edge of the pool.

On the cadre’s whistle, you let go, pushed off and swam underwater to the other side. One side went low and the other went high. The low guys tried to stay as high as they could so they used less energy. The perils of this drill included running head-on into an opposite swimmer, causing both of you to lose all momentum, or having the guy next to you take your mask off with his push-off arm stroke.