>>> but first, these messages.
>>> back now at 7:44, kicking off special series we call "silent warriors." an inside revealing look at the u.s. navy s.e.a.l.s and their secretive and dangerous work. nbs's chris jansing is here with more.
>> reporter: you know the navy s.e.a.l.s have this almost mythical, james bond-like reputation. they pride themselves on keeping a low profile. over the last six months, we were given amazing access to active s.e.a.l.s and their training bases. we wanted to find out what makes these guys tick. this morning, a look at their pursuit of extreme fitness. and suffice it to say, s.e.a.l. train something not for the faint of heart.
>> everyone pulls their own weight.
>> reporter: it's only day two of s.e.a.l. training and you can see the exhaustion, feel the intensity of what's known here as extreme fitness.
>> it's always hard. you work harder than the next man.
>> reporter: it's extraordinarily comprehensive training. because to become a s.e.a.l., you have to do it all. from shooting high-tech weapons to hand-to-hand combat. from underwater dep demolition, to parachuting into war zones. extreme fitness is absolutely critical to pulling it off.
>> it's all about feeling comfortable in an uncomfortable situation. and still being able to pay attention to those details that can get you killed.
>> reporter: coronado, california, is ground zero for whipping s.e.a.l. candidates into the best shape of their lives. at the notoriously s.e.a.l. training camp called buds.
>> you don't know how far you can be pushed or how far you're capable of going until you get through of this.
>> reporter: think two hours of boat training in cold water is hard? the day is just getting started.
>> come on, guys! keep it up!
>> reporter: how about another two hours, carrying 200-pound logs or inflatable raftings. even to get fed, they have to run. six miles a day, back and forth, to the chow hall. almost every s.e.a.l. candidate has been a successful high school, college or even olympic athlete. that means that their skills and strengths are often sport-specific. so they're trying to push them to a new level of fitness at every level of training. class number 280 started with 117 candidates. already, by day three, 19 went d.o.r. dropped on request and turned in their helmets. typically, only 25 or 25% will make it through and buds is just a taste of what it means to be a s.e.a.l.
>> every one of them will probably be colder than will probably seem wetter, more tired and more miserable than they experienced through training.
>> reporter: veterans s.e.a.l.s och play as hard as they work.
>> my thing right now is ironmans.
>> reporter: endurance sports are a s.e.a.l.'s idea of fun, from ultramarathons to triathalons to entire teams of competitive mountain bikers.
>> you can call it an occupation, but it's certainly a way of life.
>> reporter: a way of life that starts with an extraordinary ability to ignore pain, exhaustion, cold water. it may be hard to believe after watching that, but s.e.a.l. instructors will tell you that most recruits don't drop out because they aren't fit enough, but because they can't take the pressure. so navy psychologists have been studying the science of mental toughness and sharing their research with u.s. olympic coaches and athletes. i'll have that part of the story tomorrow.
>> i get tired just watching them.
>> reporter: some of the guys get temporary bald spots from carrying the rafts on their heads.