Many prospects who sign up to be Navy SEALs have the physical ability to complete training. But, as NBC’s Chris Jansing reports, the crucial quality they must have is mental toughness.

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>>> after this.

>>> we're back now at 7:44 with more of our special series "silent warriors." it's our exclusive look at the lives and the training of u.s. navy s.e.a.l.s. known for their physical grit and determination, nbc's chris jansing discovered that's not the hardest part of the job. chris, good morning to you.

>> good morning, matt. well, one thing you learn very quickly at s.e.a.l. training, it's not always the biggest and baddest looking guys who get through. since 9/11, they have been the go-to operative against al qaeda, the taliban. they're experts on urban warfare. to do that, they have to have a high level of mental toughness, and their training is designed to weed out the weak. in a darkened room in coronado, california, a blackout hood comes off a navy s.e.a.l. he has to react to any threat that faces him quickly and efficiently. it's a critical test of mental toughness.

>> you might be getting hit the second the hood's off. you might be feeling a blow from an adversary. the hood room forces a s.e.a.l., much as he does on any combat mission, to be in a situation that he has to do the right thing.

>> back up!

>> back up!

>> reporter: for s.e.a.l.s, mental toughness is the number one indicator of success, and they build it through constant training challenges. it could be instructors trying to break a young recruit.

>> what are you smirking at, you little punk?

>> happy to be here, instructor!

>> shut up.

>> reporter: or a scuba drill disconnecting the air supply to see who will panic. or testing mind over matter to endure brutally cold water in alaska. even training that is mostly physical, like the obstacle course, will often have a mental toughness component. this cargo net obstacle, for example, isn't just an issue of are you fit enough to climb it, it's also 40 feet high, so there's a little issue of vertigo. the navy has been doing cutting-edge research on mental toughness, developing a written test that helps weed out candidates who aren't likely to make it through training.

>> for the first time, to my knowledge, we're able to predict from the neck up some of the science of mental toughness, as it were.

>> reporter: and they're studying ways to control the human response to high-stress situations.

>> panic, fear, doubt. the minute we teach our candidates to be able to control that, their confidence goes up because they don't have the same fear response you and i might have.

>> reporter: now that research and experience is being shared with u.s. olympic psychologists. how does a s.e.a.l. on a mission, how does an olympian in the olympic moment make training count when it counts most?

>> we have to be primed and ready for an elite performance activity.

>> get it together.

>> reporter: members of the olympic water polo team even came to coronado and were put through some mock hell week drills. after 4 1/2 hours, they were exhausted.

>> so often, the difference between winning and losing is just a fraction. and so, we were looking for whatever little piece we could grab.

>> reporter: on the playing field and the battlefield, using mental toughness to come out on the winning side. there are actually two former olympians on s.e.a.l. team one and another currently in training. both s.e.a.l. instructors and olympic coaches told me that what they all have in common is mission focus. and actually, the s.e.a.l.s are now looking at certain kinds of athletes when they recruit, and matt, we'll have more on that tomorrow.

>> no panic on your part on the rope ladder thing?

>> once i got to the point where they said, you know, stop, we want to shoot this, and i looked down, big mistake there. and there are a lot of people who freeze at the top, really physically fit guys, once they look down 40 feet, that's it, they can't keep going.

>> they would have had to chopper me out of there.

>> could you do it?

>> i don't know. not the top, no. chris, thank you.

>> sure.