Bat-Nado! 20 Million Bats Majestically Fly, Feast Nightly

Nat Geo's Casey Anderson heads to San Antonio to film Mexican free tail bats.
5:50 | 09/13/13

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Transcript for Bat-Nado! 20 Million Bats Majestically Fly, Feast Nightly
Forget shark NATO get ready to behold a nonfiction -- NATO or creepy creatures crammed into one dark dank -- suddenly. Rush out any whirlwind forty million wings. Come from vampires bloodsuckers. -- guys have definitely had an image problem but -- turns out they are doing us a big favor every time they feed and there's so much more. Than the stuff of Halloween nightmares ABC's Ryan Owens travel to Texas. Who witnessed the feeding frenzy. Welcome to the -- NATO. That's right to own real tornado of creepy creatures of all formal look freaky flyers that emerges every night from this game. Look at that. And you feel the -- flower. Itself. Very -- in the middle. You were watching twenty million yes to 1 PM million Mexican free tail bats the world their way out of a single gave -- to hunt for food. -- that it -- -- -- the way that they're climbing out of this -- I'm gaining altitude like an airplane gain altitude -- up. You can see they keep their form they can go off into river off thirty miles away hearty. Hundred tons of bugs every. Our guide for this tour of the bracken bat -- just north of San Antonio. -- -- Casey Anderson. Host of America the wild on Nat geo wild. I -- tracking predators. This guy has traveled the world and seen some of its most dangerous animals. And says he's never seen anything like this it's the largest concentration of mammals. And it took this -- overwhelming to be. You know amongst all these bats so many -- make this journey together every night they even show up on the local radar. And as many as were saying. Right now this goes on for hours and hours just a steady stream. In this massive load on her three hours sometimes in his shoes you how many bathroom that connected -- it's. Is it blows my mind and as close as we are here. You're telling me I'm in no danger of one of these it's me and I have known only endanger news who wants -- -- you might feel like you didn't marine Don nice thanks to -- and that's the answer to me OK let me very glare. We couldn't go inside this cave because of the dangerous levels of CO2 from all of the bats well guano. This is crazy but Casey did go inside a similar case and it was truly the stuff of -- Alex box palm ceiling. Yeah so tricky. And he argues despite all of those childhood nightmares and cheesy horror movies like the aptly named -- We need. Really should the lights down. Nothing -- -- here ambassador super essentials. Every ecosystem and -- -- back to their action -- are best friends middle of summer hot here in Texas people think there -- a lot of bugs right now if we -- of these guys. Would be in real trouble. I can't imagine. What it would be like if the bats -- -- hundred tons of bugs every night that would not be going away we would live. In a world covered box next to me something actually can be afraid of -- spirit and -- absolutely. It turns out these bats have a lot to be afraid up as we waited for them to -- that night. We noticed skunks and raccoons gathering around the cave entrance hoping at least a couple of the twenty million fall out of the sky. Snake show up at the but they too. And they don't always wait for those bats to hit the ground Nat geo -- this snake snack -- wondering right out of the air here. -- is down right there. Even if the bats do make it high in the air there's another predator waiting for them check out this hawk way. Around dinnertime. That's not to say some bats aren't scary Casey went to southern Mexico in search of vampire bats. Yes they do exist and just like Dracula they only -- one thing -- weird animal. It's like the but they don't like human. In fact they usually eating livestock coroner wild animals found out about her -- something else vampire bats like. The heat so for now they are only blood sucking he -- down in Mexico here's some speculation is that as the climate. And is that you know many species -- -- start -- And there is indications that vampire bats are coming north you know and here texas' cattle country as funny if -- of the vampire bats to eat. And that could happen. Until then let's enjoy the harmless variety. And there is something mesmerizing about watching these animals leave the so called bad NATO. And trail off on their nightly commute almost like they're on board a train now we can't see. I think this is absolutely beautiful and -- as you know. Birds people would love it let's not bats but I think it's equally as beautiful and maybe in some way but more important. Their commute is a long one they fly as far as thirty miles in search of those and sex and they returned just before sunrise. Cramming twenty million bats back into the cave. Is something to -- two. There's like an -- ballet took us and I can't believe -- using that -- I think you're spot on ballet and respect to balance between million of anything. -- -- -- -- -- -- -- Very much and that incidents and it's -- dance that they have to do every night to get where -- -- -- -- Nighttime is right time -- the back. One of the greatest shows on earth courtesy of mother nature. I'm Ryan Owens for Nightline at bracken bat cave in Texas.

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

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