Anacondas: Tracking Elusive Giants in Brazil

Legend has it these Amazonian giants drag off children and pets in the night.
3:00 | 09/22/12

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Transcript for Anacondas: Tracking Elusive Giants in Brazil
The anaconda is one of the most fearsome beasts on earth. A giant snake that lives off its own incredible strength. And rose to pop culture fame as the villain of a cult classic film. So, our team went knee deep into the remote waters of brazil to chase need snakes, aided by one man who has made it his life's work. Here's abc's matt gutman. Reporter: Underwater, faced with the world's biggest and deadliest snake, the anaconda. Those beady, lifeless eyes. The endless scaled form, over 200 pounds of muscle, sliding a few inches from my face. Its head, the size of an anvil. It's the stuff of nightmares, horror movies and native myths. Legend has it they drag off children and pelt pets in the night. And to my horror, the legend has to be true. Why do people fear them so much? Well, there have been records anacondas try to get children. Reporter: Good reason. Yeah. Reporter: That's lawrence, who spent months tracking the world's biggest snakes, diving with them and filming them for nat geo wild's secret brazil series. A peek into the untamed world of brazil's savannahs. There, the dry season pushes animals to desperation. The feeding frenzies of the piranha, the cannibalism of the crock's cousin. It's a place where anacondas can bite off more than they can chew. Lawrence took us on a journey to the anaconda's heartland. Remote bonito. Hey! Bonito. Welcome. Reporter: We unloaded the boats. The stream cool and clear. I'm kind of hoping we don't find one so I don't have to go in the water. Don't say that. Come on, man. Reporter: You get to defend yourself with that and I have this. No, this is not what you think. Reporter: And our date with an anaconda approaches. What are we looking for? If we see a thick branch that swims, that's the anaconda. Reporter: We found the stream where just months before, lawrence found this monster. The head was giant. As thick of a dive tank. That was a dinosaur. It was a snake that when I look at it, I say, "wow." Reporter: Our local guide is tracking these waters for 50 years and seen these snakes take down a capybara. Watch in this video. An anaconda crushes the capybara's bones and begins to swallow. But they only feed a few times a year. They feed off pigs, dogs, chicken. So, the natives don't like them. Reporter: Soon enough, they spot one. There's a small one. Ready? Reporter: No, I'm not ready. This is insane. But seeing them in the brush isn't good enough. When the boat gets too close, the anaconda launches itself into the water. We follow it for a few seconds -- but with the flick of its tail, it's gone. What's your face nation? You keep coming back here. You dive with them. I think it's a monster, 26 feet. Anaconda is one of the most charismatic and fay mouts animals from here -- Reporter: Charismatic? Oh, absolutely. You look into his eyes and he's strong. Like the indigenous people, they believe they're not a snake, they're an entity. A lot of myth relating anaconda to the creation of the world. Reporter: What about that tongue? Yeah, sometimes it's scary, like, when it's here. Reporter: We hop in the zodiac. 50-foot anaconda. Reporter: Minutes later, we spot it. See the head? Reporter: A giant, closer to 17 feet. It's now tense as we get into the water. Even if he's under your legs, don't move. Reporter: No matter what, don't move? Freeze. Reporter: The first was quick and illusive, but watch this one. All 17 feet of it slithers from its snoozing place. Under water, you see the giant head coming towards us. We're holding our breath that this anaconda keeps coming. I reach out my hand, tiny, compared to that massive body. Between them, they have 40-years plus experience filming underwater. Their shots are steady, calm. Mine? Frenzied. But the anaconda isn't. It's calm. Dissolving effortlessly through the murk. And silently, the giant disappears. I'm matt gutman for "nightline," bonito, brazil. Wow. "Secret brazil" premieres SUNDAY, SEPTEMBER 23rd. Thanks to matt and the anaconda.

This transcript has been automatically generated and may not be 100% accurate.

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