[ chanting ] >> reporter: every day begins with physical training, miles of swimming, running, hundreds of sit-ups, push-ups, all before the day's real work begins. even more important they are preparing the body is preparing the mind. case in point, an exercise called drown proofing. >> candidates' hands are tied behind their back, feet tied together and you are thrown in the pool. you better not panic. control your breathing, your heart rate. they tell you day one at b.u.d.s., mental toughness, not physical toughness. what's between your ears keeps the body going. >> reporter: and then comes hell week. six days, little sleep, subme e submerged in frigid water or running hundreds of miles.