BONITO, Brazil, Sept. 22, 2012 -- As I held my breath underwater, the 200-pound anaconda slid leisurely beneath me -- inches from my face. The endless, scaled form with a head the size of an anvil is a creature from nightmares (including my own) and native myths.
Legend has it these Amazonian giants drag off off children and pets in the night.
I was forced into the water by Lawrence Wahba, a swashbuckling Brazilian filmmaker, who spent months tracking the world's biggest snakes for National Geographic's "Secret Brazil." He followed anacondas through the murky waters, capturing a glimpse into the untamed world of Brazil's savannahs.
The savannahs, smack in the heart of the South American continent, are more arid than the famed Amazon, and the dry season there pushes animals to desperation: There are the feeding frenzies of the piranha, the cannibalism of the crocodile's cousin, the caimans. And it is a place where anacondas can bite off -- and swallow -- more than they can chew.
Wahba took "Nightline" on a journey into the anaconda heartland -- in remote Bonito, nearly 1,000 miles away from Rio.
From a ride on a single-engine plane into a rickety truck, carrying even ricketier boats, we rode though endless ranchland flanked by streams set aside as natural preserves.
Then we found the stream, cool and clear, where just months before Wahba found a monster 25-footer -- possibly the largest anaconda ever filmed. It was as wide as a truck tire.
"So they're really, really sensitive," Wahba said. "We need to be really quiet -- whisper, talk very, very slowly and, if we see one, be very, very silent."
Our local guide was Juka, who has tracked these waters for 50 years and captured never-before-seen video of the giant snakes devouring animals like capybaras -- the world's biggest rodent, which can weigh in at 100 pounds -- crushing their bones and swallowing them whole.
"The good news is they only eat two times a year or sometimes three times a year, and they are not aggressive," Wahba said. "Even the big ones will try to avoid us."
Soon, he spotted one "only" about nine feet long. We jumped in with it and watched it glide effortlessly through the water. It slithered away into the brush. They kept calling it the "baby snake," but it was half as wide as the stream we were boating in. Wahba seemed to have great respect for these creatures.
"You look into his eyes, and it's strong," he said. "The indigenous people, they believe it is not a snake, it's an entity. There are a lot of myths relating the anacondas to the creation of the world. It's interesting."
But as we cruised along, a different sort of trouble struck. Our boat became stuck and Juka told us the engine overheated, so we were forced to walk upriver to track the anaconda without a boat. Eventually, we gave up and paddled downstream back to the truck.
But as it grew dark, Juka got a phone call. There were fresh reports of a snake that locals were claiming was 15 feet long.
So we hopped into a different, rubber boat. Minutes later, we spotted it: a giant, closer to 17 feet long.
Again, we hopped into a boat, and pulled on our dive gear. It was tense as we prepared our camera equipment and I fiddled with a mask and snorkel.
"Even if it comes between your legs don't move -- freeze," Wahba warned me.
Diving underwater, I saw the anaconda's giant head coming toward us. As we held our breath, it just kept coming.
Tune into "Nightline" tonight at 11:35 p.m. ET to find out what happened.