Reporter: Liz Hayes
Producers: David Alrich, Stephen Rice
Witness an underwater drama that is quite simply mind blowing.
A face-to-face encounter with a four-and-a-half metre wild crocodile.
There are no cages here, no guns, no tranquilisers, just one brave diver and a giant prehistoric man-eater.
Dangerous? You betcha!
But this isn't some kind of extreme sport. This is scientific research in the scariest laboratory on earth.
LIZ HAYES: You're watching an underwater meeting of the most extraordinary kind - man coming face to face with a very alive, very aware, crocodile.
ADAM: There's no doubt your brain suddenly starts to think, "Adam, what's going on here? We're in the water with a wild crocodile."
LIZ HAYES: Australian zoologist Dr Adam Britton is studying crocs in a way that no-one ever thought possible.
ADAM: Your entire body is telling you that there's something wrong with this situation - get out now.
LIZ HAYES: Because when things do go wrong, it can be deadly.
ADAM: I suppose it's always possible that a crocodile in a situation like this might suddenly figure out that perhaps we might be worth tasting, whether we're worth chewing on.
LIZ HAYES: And for those who think you are crazy - this is just madness - what's your answer?
ADAM: I think that anyone thinks that I am crazy - maybe they're right. But at the end of the day, well, I don't know.
LIZ HAYES: The setting for this crazy tango between man and one of the world's most vicious predators is the Okavango Delta in Botswana - home to the Nile crocodile. It's also home for wildlife documentary makers Brad and Andy Besterlink.
BRAD: Welcome, nice to meet you.
LIZ HAYES: Brad grew up here, and he and his wife, Andy have been diving in the Delta for years - but always studiously avoiding the local crocodiles.
ANDY: The first time we saw a crocodile underwater was totally by mistake. It found us, we didn't go looking for it. It just came out of a cave that we didn't even know was a cave, it was just a sort of dark area, and it just came out, terrified, in between Brad and I, and we both - you know - this big crocodile just swam right between us and settled on the sand next to us. So that's the first time we saw one.
BRAD: So there's a croc lying there, facing away from us, and we're sitting there watching it, and it's like, well…
LIZ HAYES: What do we do?
BRAD: Now what do we do, you know? And you just lay there, you know, and you say well, let's go have a look.
LIZ HAYES: Really?
LIZ HAYES: You thought, let's go and have a look?
LIZ HAYES: Brad and Andy had stumbled upon an extraordinary discovery - that the ferocious crocodile could be a totally different creature when underwater.
BRAD: There are a number of things that really changed our perception of crocodiles. We didn't realise they were so timid under water - definitely didn't realise that by nature they're actually quite timid.
ANDY: They're not these mindless killing machines.
LIZ HAYES: This reptilian revelation is thanks to the unique conditions found in the Okavango. The world's largest inland delta is stinking hot in summer. But come winter, an icy chill arrives, dramatically slowing the metabolism of these cold-blooded killers.
BRAD: Diving with crocodiles essentially couldn't have happened anywhere else in the world that I know of. It's the one place where the temperatures actually drop right down to a point where the animal virtually shuts down.
LIZ HAYES: So they're not so hungry.
BRAD: They're not so hungry, so they're very slow - they're very sluggish.
LIZ HAYES: For Darwin-based researcher Adam Britton, the prospect of actually diving with crocs was an unbelievable breakthrough. So when Brad and Andy invited him to Botswana, he jumped at the chance - albeit with a healthy dose of apprehension.
ADAM: I have to admit, when I was first putting on my gear, my scuba gear, and putting on the tank, that very first time when I was in the boat, my heart was in my mouth, because you think, "What if these guys are wrong or what if what if the chances of this going wrong are sufficiently high, that I'm going to be the guy that actually has the unpleasant encounter?"
LIZ HAYES: In freezing winter temperatures, we head out to witness this unbelievable business of diving with crocs. But first we have to find them.
BRAD: It's a bit of a cat-and-mouse game, when he goes in, it's a real challenge to find him.
LIZ HAYES: So, what are you looking for as we're going? I mean, any place here looks to me like it could have a croc.
BRAD: One of the rules we've got with the diving is you definitely have to see the animal on the bank - so I'm looking for ones that are basking, trying to get warm.
LIZ HAYES: We find one. That's a dentist's delight right there. And this is when things get really tricky. When we approach, it disappears, and tension onboard begins to build.
ANDY: Adam, we've got to stick together, OK? That's the most important thing.
BRAD: I've given us a long lead-up, so if there's a problem we can all get out.
LIZ HAYES: This is quite a critical moment - the time between spotting a croc and getting into the water. Brad estimates he and his team have something like six minutes before a croc starts to view them as prey.
ADAM: They can see, they can hear, they can feel vibrations. They've got a good sense of taste, obviously smell when they're on the surface as well. They're kind of 'bite first, ask questions later' kind of animals.
LIZ HAYES: But when man and crocodile come face-to-face, it is an extraordinary moment. Who do they think you are down there?
ADAM: We could be a large fish that's too large for them to catch, we could be a hippo that they want to stay well away from, we could be another crocodile, we could be a log drifting down the river. I mean, they know it's not necessarily something that's going to threaten them, and they don't see it as food because it's not the kind of situation that they normally associate with feeding.
LIZ HAYES: Since their first encounter five years ago, Brad and Andy have dived hundreds of times without so much as a scratch - although there have been some heart-stopping moments.
BRAD: Crocs have swum into us.
LIZ HAYES: Swum into you?
BRAD: Swum into us.
LIZ HAYES: Hit you?
BRAD: It happened, swim straight into the port. We had them swim literally chest and disappear underneath you, you know?
BRAD: It's actually quite funny - the easier ones to interact with are actually the bigger ones, because they're less threatened by whatever is in the water.
LIZ HAYES: Did you ever think that you'd be cuddling up to a croc?
BRAD: No. It's a weird thing to see that side of them. It's just totally, totally different.
LIZ HAYES: There is another way to get close to Botswana's crocs - at night, when, believe it or not, they're more easily spotted.
ADAM: That call means 'help, help, I'm being attacked.' And other adults will come in and rescue whatever is attacking the baby croc. There's a distinct possibility as well the adult female, and possibly the male responsible for this guy is still hanging around.
LIZ HAYES: Adam is conducting a three-year genetic study, which means he needs to examine as many Nile crocodiles as possible - and in some very intimate ways. I am not sexing a crocodile, I'm here to tell you. Traditionally, this is how most tissue samples have been taken - hard enough when it's a baby, next to impossible when it's a fully-grown monster. But now, Adam believes he can do what no-one thought possible - take DNA from the big crocs underwater.
ADAM: The first time I got close enough to the tail of a crocodile, I thought, well let's just try picking up the tail - and the crocodile was completely oblivious to this, and that was what gave me the thought. I thought well, if I can pick up the tail, I can take a tissue sample from one of the scoots on the end, and it won't hurt the crocodile because essentially dead skin - it won't feel it. I should be able to get away with it. So that was the mission this time.
LIZ HAYES: You should be able to cut its toe nails next.
ADAM: Yeah, you probably can.
LIZ HAYES: So the mission begins.
ANDY: Have you checked how sharp those scissors are? Have you tried cutting something?
ADAM: Yes, they're very sharp.
ANDY: Because you don't want to be hacking.
ADAM: Well, the sharper it is, the less the croc feels it.
LIZ HAYES: The team spots a croc that could be Adam's first patient. It is an astonishing sight - a scientist stalking his 4.5m prey. He pulls out his tweezers and scissors - and misses.
ADAM: I just go into position. I was just about to snip the tail, and the croc moved off
LIZ HAYES: Adam tries again, and in what is an amazing feat, snips the first sample ever taken from a crocodile underwater.
ADAM: Well, we did it.
BRAD: No, it was amazing.
ADAM: We did it big time.
BRAD: No, big croc, yeah?
ADAM: Oh wow.
BRAD: Nice croc, I mean, that guy must be 4m - in excess of 4m. Really nice croc.
ADAM: Oh, he was absolutely stunningly beautiful - this massive, massive crocodile. Picking up the tip of his tail and I was like, "OK, moment of truth, and snip," - and he felt it.
LIZ HAYES: I can't believe that you're doing this.
BRAD: No capturing, no torturing, no rolling around.
LIZ HAYES: You're shaking hands with a croc underwater. Amazing. Let's have a squiz.
BRAD: Make sure I don't drop it. I'm shaking because it's so cold down there.
ADAM: There's a tissue sample from a four-metre Nile crocodile.
LIZ HAYES: Weren't you just a little terrified?
ADAM: No, absolutely not. I was just blown away.
LIZ HAYES: You mad bugger.
ADAM: Not the kind of thing you're going to forget any time soon.
LIZ HAYES: Now, if you're wondering - no, I'm not a diver, and I think you've got to be mad to dive with crocs. And if you are a diver and thinking you'd like a slice of this action, well, forget it. Croc-dive tourism is not on Brad and Andy's agenda.
BRAD: I think the intention's different. I think if you made it into a tourism thing, the reason for doing it is wrong, and that can get you into some big problems.
LIZ HAYES: That might be when the croc wins.
BRAD: Exactly, that's when you will have problems.
LIZ HAYES: This is risk management in the extreme. With two young kids, Brad and Andy are forever calculating the odds. And for Adam Britton, the benefits to his research outweigh the potential threat to his life.
ADAM: What's interesting about diving with crocodiles is people's reaction to it. I mean, most people think that's crazy, that's totally insane. But when you're trying to study an animal, you have to go into the environment, you have to look at it from the perspective the animal lives in. I mean, the reason I'm doing it is because I'm convinced, knowing what I know about crocodiles, and knowing what I know about everything that we do here, that we can get away with it.
LIZ HAYES: And 'get away with it' is what Brad and Andy hope to do for a long time to come. If either of you were to be taken by a crocodile, would we be right in thinking it was inevitable?
ANDY: I think a lot of people think so. Maybe it's a matter of time, maybe we're going to come across that personality, that particular psycho croc on that day. So you do question it, but you've got to go by your own judgment to an extent - you follow your heart and we are not cavalier, as I said, with our lives.
BRAD: But we have written wills.
LIZ HAYES: I'm glad to hear it.