THE green anaconda is the world's largest and heaviest bodied snake. It spends most of its time in the water which allows it to support its body weight. And despite the fact I have filmed anacondas many, many times I'd never filmed them in the water before.

So I found this location in Brazil where there's very clear cool-running streams and I went there with all my dive kit and went up and down these rivers searching for a big snake. When we finally saw one it was like having this great big tractor tyre lying on the bank. It was as thick around as my thigh.

And as we approached it slipped into the water and we got all our dive gear on and went after it. The bottom of this swamp was like something out of a Hammer horror movie. It was gloomy and gloopy and you had all these tangled, twisted tree roots going down into the water.

All of a sudden I heard my cameraman say, "Steve. Steve. It's here." I swam over and this huge, huge snake with completely white ghostly eyes was dragging itself along the bottom and it came up and it sniffed around using its tongue and then it swam straight into the camera.

Encountering this incredible monster of a serpent on the bottom of this primeval swamp was an experience that made me feel like some kind of aquatic caveman experiencing this unbelievable dinosaur in an incredible prehistoric place.

We were in the water with the anaconda for an hour. Eventually it disappeared into a tangle of tree roots and the encounter came to an end. But for those few moments it was utterly spectacular.

How can I bring myself to do it? I think in a lot of cases it is experience and knowing how far you can take it and still be safe. Sharks are the classic example. A lot of people think just getting into the water with sharks would be suicidal, but I've been diving with sharks since I was 12 years old. I started with the small stuff and worked my way up.

You get to realise there needs to be good visibility and you need to be diving at the right time of day for that particular shark and not at the time when it's at its most predatory. And when you get everything right you can swim outside a cage alongside a great white shark and it will totally ignore you. It will have no predatory interest in you whatsoever. So it's a question of building up that experience over time

As for snakes, it does appear to be that there is no absolute word on whether our fear is inbuilt. Is it there in what Jung called the collective unconsciousness? Or is it something we learn?

But if I do animal introductions for young kids – three years old, four years old – they will quite happily handle a snake. Come back a couple of years later and they appear to pick up on the signs of their friends around them and even more so the adults. If the teacher's screaming: "Ah, it's a snake," then the kids will be much less likely to want to hold it. So from my personal experience I would say we have a fascination, but not necessarily an inherent fear.

Steve Backshall's Wild World theatre tour visits Edinburgh's Festival Theatre tomorrow and Horsecross, Perth on Monday. To book tickets visit stevebackshall.com