The following script is from "The Nile Crocodile" which originally aired on March 24, 2013, and was rebroadcast on July 27, 2014. Anderson Cooper is the correspondent. Michael Gavshon and Paul Bellinger, producers.
Of all the different species of crocodiles in the world, Africa's Nile crocodile is the most dangerous and deadly. They can grow up to 20 feet long, weigh as much as a car, and bite as hard as a Tyrannosaurus Rex. Crocodiles are prehistoric creatures that have been around since the time of the dinosaurs, but we still don't know a lot about them because studying them up close on land is treacherous, and underwater was always thought to be impossible.
The Okavango Delta has been called one of the last Eden's on earth. The hundreds of miles of winding waterways and untouched islands are home to some of Africa's most exotic and enchanting wildlife. It's also home to thousands of Nile crocodiles. For the last six years, Brad Bestelink and his wife, Andy Crawford, have been risking their lives filming these man-eaters in the most daring way imaginable: following the crocodiles into their underwater lairs.
It is a dark and foreboding world down there, visibility is sometimes only a few feet and you can't even see the crocodiles until you catch a glimpse of their long rows of razor sharp white teeth.
Brad Bestelink: We were next to a ledge. And this crocodile swam out. And actually swam between us. And then settled on the ground next to us.
Anderson Cooper: What first went through your mind?
Andy Crawford: Well, just lots of bubbles?
Brad Bestelink: Bubbles.
Andy Crawford: And just panic.
The panic was understandable. Nile crocodiles are Africa's largest and most feared predator but surprisingly this one didn't attack. Brad and Andy have been getting closer and closer to these creatures ever since.
Brad Bestelink: You do get a different sense of them. They look very beautiful underwater. They're dappled and gold and black. And you see them as more timid I think. Beyond the teeth and the terror there's this incredible creature that is actually an amazing animal in its own right.
Anderson Cooper: You actually think they're beautiful.
Andy Crawford: I do think they're beautiful. I never used to think they were beautiful. But this is a whole different view of them.
"Beyond the teeth and the terror there's this incredible creature that is actually an amazing animal in its own right."
This is the view most people have of Nile crocodiles. Patient and stealthy killers, they grab their prey, drag them into the water then drown and dismember them. And it's not just animals they eat, hundreds of people in Africa are killed each year while bathing, laundering clothes, or fishing along the waters' edge.
Nile crocodiles are now protected in Botswana, but Brad and Andy believe more needs to be known about their behavior in part so that humans can better avoid them. They've invited Dr. Adam Britton, an Australian zoologist, to dive with them.
Anderson Cooper: When you first heard about what they were doing here, what did you think?
Adam Britton: Look, I'll be honest. When I first heard about this, my instant, immediate reaction was "That sounds crazy."
Doctor Britton has been studying crocodiles for almost 20 years.
Adam Britton: I describe crocodile like Ferraris. They're just extremely finely honed creatures. They are just perfectly adapted to do what they do. They're, you know, the smartest of all the reptiles.
Britton is building a genetic database on Nile crocodiles in the Delta to better understand how to protect them. For years the only way to study them up close was to capture them.
Brad Bestelink: Croc on. So once I've got him by the mouth...
It is difficult, dangerous work.
Adam Britton: Sit on him, get his legs back. Pin his legs between his knees as well. He's got no leverage.
Anderson Cooper: So what are you doing now?
Adam Britton: I am just going to cover his eyes so that he can't see what we're doing.
Anderson Cooper: So he's not injured at all?
Adam Britton: No, no, he's not injured at all, apart from his pride perhaps.
This crocodile is not sedated. It's simply trying to conserve its energy.
Anderson Cooper: Why are you doing this?
Adam Britton: If we can get a sample of all the DNA from every single crocodile across the Delta then we can start to build up a picture then of exactly, not only where these crocodiles came from, but how they're moving within the Delta.
Anderson Cooper: Because right now you don't really know that?
Adam Britton: No one knows anything about that.
Anderson Cooper (on camera): When you actually see the crocodiles up close, there is a beauty to them. Often in pictures they're covered in mud. They look very drab. But up close, you see the variety of color not just on the top, but also on the bottom. And to the touch it's really-- there's a softness to them, particularly on the feet like this. The claws are about an inch, an inch and a half. But the pads of the feet are actually incredibly soft.
Capturing crocodiles is stressful for the animal and for us. Putting them back in the water is just as hard.
Adam Britton: Keep pressing down Anderson on the top of the skull. That's good.
Anderson Cooper: Noose ready to go.
Adam Britton: OK. Three, two, one go.
Diving with Brad and Andy has given Dr. Adam Britton a whole new understanding of crocodiles and their underwater world.
Adam Britton: You're in the water. You've got the current washing over you. You can feel the changes in temperature. And you suddenly think, "This is what it's like to be a crocodile. This crocodile is experiencing these same things."
Britton has actually begun to take DNA samples from crocodiles underwater, cutting off pieces of their tales and, incredibly, they don't seem to mind.
Diving with Nile Crocodiles is only possible in the winter months when the water is chilly, and the animals are sluggish. These cold blooded reptiles are far too dangerous to dive with in the summer.
"You're in the water. You've got the current washing over you. You can feel the changes in temperature. And you suddenly think, 'This is what it's like to be a crocodile. This crocodile is experiencing these same things.'"
Brad Bestelink: The crocs are much more active. They're much more inclined to want to predate. You know, I don't--
Anderson Cooper: Predate, attack.
Brad Bestelink: Attack, yeah. They want to go and eat something.
Anderson Cooper: So, two months from now, three months from now, you would not dive in these waters.
Brad Bestelink: No. No. No, and I don't. I don't want to die. Make no mistake. I do this because I get an understanding as to how these predators work.
Brad and Andy offer to take me diving with them, explaining it's crucial to get off the surface of the water as quickly as possible because that is where crocs attack.
Andy Crawford: That's the most important thing. Because as soon as you're underwater we believe the crocodiles don't know what we are. They don't recognize us as prey.
Anderson Cooper: You say, "We believe." Do you know?
Andy Crawford: We don't know it for sure. We can never know how they're perceiving us. We're trying to establish how they perceive us.
Anderson Cooper: You're not really building my confidence here by saying you're not sure. What do I need to know before going in?
Andy Crawford: Well, you need to know we believe you're safe. With all that uncertainty, we believe you're safe.
Safe? Take a look at a recent encounter they had with a crocodile.
Brad Bestelink: You see how close he comes to me?
Anderson Cooper: And look at the eye.
Brad Bestelink: Yah.
Anderson Cooper: And look at those teeth. Those are huge.
Brad Bestelink: They are.
This crocodile was 12 feet long and weighed about 14 hundred pounds
Brad Bestelink: And there's a diver. And watch what he does.
Anderson Cooper: Oh my gosh.
Anderson Cooper: So but because the croc's moving, it doesn't even really sense that diver there.
Brad Bestelink: It didn't even know that he was there. And you'll see how it just goes, it hits his light and squashes his light.
Anderson Cooper: So it just thinks that some--
Brad Bestelink: Yep. Yeah. Yeah.
Anderson Cooper: --debris or tree or something? That's amazing.
We set off early the next day. It's an hour up river to a spot that has a lot of underwater caves. Three divers will go in with me: Brad, cameraman Richard Uren and Andy. She will be the safety diver watching our backs.
Male voice: I'll let you know that there's a croc if I see it first.
Andy Crawford: The sign of crocodile is that. That's the sign--
Anderson Cooper: It's the international sign for crocodile?
Female voice: That's the sign for-- well, it's a sign for crocodile.
Anderson Cooper: OK, OK, good.
Female voice: Brad does this.
Anderson Cooper: I didn't learn that in SCUBA school, they didn't teach that.
Andy Crawford: We're gonna give you one of these to dive with. It makes you feel better. It also gives you some barrier--
Anderson Cooper: Makes you feel better? That's really all it's for is just to make me feel better?
Andy Crawford: Well, mainly that, and actually-- to actually anchor yourself in the current.
Anderson Cooper: Because no matter what, you do not want to drift onto --
Andy Crawford: You don't want to drift onto the crocodile.
As soon as the crocs see our boat they disappear. We hope they've gone to the bottom to hide in underwater caves, but they might still be floating near the surface waiting to attack.
It's a very strange feeling before you go diving because you know there are crocodiles in the area but you don't see any on the surface. The problem is as the boat comes in any motion on the surface does tend to attract crocodiles so you want to try to get here and into the water and to the bottom as quickly as possible.
We suit up, do our final checks and then take the plunge.
Brad Bestelink: Anderson good? OK.
We get to the river bottom as quickly as we can. It's only about 15 feet deep. Thankfully, the visibility is good and we find ourselves in a stunning underwater garden with overhanging ledges, walls of papyrus, submerged trees and lilies.
Anderson Cooper: We know there's at least one crocodile in this area because we saw the ripples on the water. We believe it's gone into a nearby cave system and we are going to go into the caves right now to try to see if we can find it.
It's eerie and intimidating down here. The only light comes from our cameras, and it's easy to lose your way.
Brad signals that he sees a crocodile. At first I can't see anything, but then out of the darkness, on the floor of the cave, just as Brad warned, I see that gleaming row of white teeth.
Anderson Cooper: To finally see one. It's amazing there's a beauty to it, but it's also incredibly intimidating. You really have a sense when you're so close to it of just how strong it is. And it looks right at you, and you know and it knows that it could attack you at any moment. And there is nothing you can do about it.
The crocodile disappears into the darkness. We push further into the cave. It gets narrower and more claustrophobic as we move deeper into the gloom. Then, lurking on a nearby ledge there's another crocodile.
Anderson Cooper: This crocodile is about nine feet long. Its tail though makes up half its length. Crocs have the amazing ability to actually slow their heart rate down and they can close off one of the valves in their heart, to stop the blood flow to some of its organs and allows them to stay underwater for hours at a time.
Anderson Cooper: It's amazing how close the crocodile is. You can't tell if it's watching you or not.
Suddenly the crocodile backs away. It's not taking its eyes off me. I have no idea what it's going to do. My heart is pounding. Neither of us moves. Then with a flick of his tail, he's off.
We move further through the undergrowth and find yet another crocodile. This time it's facing me head on. On the stick I'm holding I have a small camera and I move it closer to try and get a better shot. I know I should be terrified but the truth is it's actually thrilling.
When it finally takes off, we start following it. The crocodile is kicking up so much sand and sediment, we can't see where we're going.
Anderson Cooper: We are trying to pursue the crocodile right now, but I can't tell how large it is. Its tail is so powerful I am almost right on top of it. I can reach out right now and just touch the tail, but I am worried if do that it will somehow turn around. It just doesn't seem like a good idea. But I got to say it's so tempting.
The croc is moving so fast, we can't keep up for long. It's time to surface and find the boat.
Anderson Cooper: Wow that was amazing. I was right on top of it.
Brad Bestelink: Hey?
Anderson Cooper: I was right on top of its tail. I mean, I could have touched it.
Brad Bestelink: Yah, I know. And then he turns around.
Anderson Cooper: And then he turns around! I swear there was a moment where I thought, "Jesus, he could just attack. And there's nothing I could do about it."
Brad Bestelink: Absolutely. But did you ever feel like he was going to attack?
Anderson Cooper: No. Well, maybe a little bit actually.
I've dived with Great White sharks before, but in terms of numbers of people killed each year, Nile crocodiles are far more deadly. Once ruthlessly hunted, still vilified as mindless killing machines, we can finally observe them as they really are: perfectly evolved denizens of the dark, ancient creatures, now for the first time, fully visible in the light.