THE PURPOSE of this trip was to find the mokele mbenbe, that is the Congolese name for dinosaur. You were always told that mokele mbenbe makes a noise that, once heard, would make you lose your reason, and I wanted to know why there were all these reports like the Loch Ness monster. I thought it might be a giant monitor lizard.

I was very excited because the country is wild and unvisited . . .

You fly to Brazzaville and when you have paid pounds 3,000 in bribes, eventually you get on the steamer which goes into the forests; the Congo river first and then the Oubangui.

It is exciting as the banks come in closer towards you, there are fantastic birds, kites hanging off the stern, skimmers and kingfishers - a taste of things to came.

At Impfondo, a little frontier town, you get into a canoe and Bantu paddlers take you from village to village. Then the real unexpected things begin to happen. You see a crown eagle, a massive eagle which eats monkeys and has a mesmerising whistle, and palm- nut vultures, groups of red colobus and the odd gorilla.

I lived with the pygmies for a time, and fell in love with them. The Babinga pygmies have very bright eyes, the men are immensely muscled and the women are beautiful - little film stars, really.

They are extraordinarily hospitable and pleased to see you, and very humorous and friendly. There's laughter all the time and they make beautiful music with one little drum and marvellous yodelling cries, and it is a wonderful sight - these tiny people dancing under 200ft-high trees in the middle of the forest.

You eat very well with the pygmies: forest crocodile tastes good, we ate lots of big palm grubs and maggots, which taste like bacon.

The lake where mokele mbenbe is reputed to live turned out to be three days' walk from the nearest village, three days walking through swamp forest. Even in the dry season you have to wade with the water up to your waist, and that's unpleasant and arduous. There are fallen tree trunks everywhere, so you are stumbling all the time and getting caught in terrible vines called the wait-awhile, which have thorns on them. I lost a lot of weight, almost four stone.

But eventually bright patches of light between the trees begin to form up, and you come out of thick, thick jungle and suddenly there is what feels like this huge open space and this beautiful lake with the bluest water, about five kilometres by four and almost circular, lies in front of you.

It is very rare to see flowers in the forest, but all the way round the lake are these amazing blooms, spikes of pink flowers 120ft high. It looks like an exotic English country water garden gone berserk.

There were fish eagles out over the lake, like giant black and white moths, and the cormorants flying past looked huge, like archaeopteryx. If you are going to see a dinosaur, this is certainly the place; it felt immensely rich and old, and it really did feel like the birthplace of mankind.

I got into a tiny fishing canoe with two tough Bomitaba, a warrior caste from a village called Boha, which is said to be the most violent village in the region. They carried huge, 12ft-long spears used for hunting gorilla.

We pulled in to the bank and I realised it was going to be one of the best days of my life . . . you can tell when the animals just appear in front of you.

There was a situanga, a marvellous rare antelope with spiral horns and splayed feet for running on wet vegetation, and it looked as if it had just been lifted out of the water, because the sun was striking the surface of the lake and lighting up its belly.

Then we saw a rare bird, a tiger bittern, beautifully camouflaged in russets and brown - it looked like a bundle of autumn leaves; it makes a noise you can hear two miles away, a foghorn sound that you'd never think comes from a bird.

We got out of the canoe and I covered my shirt in mud as my back was burning like crackling, and when I looked back the whole of the surface of the lake was shimmering in this terrible heat.

I knew I would hardly be able to walk the following day because I was covered in tsetse flies, which give you the most appalling bite. But none of that seemed to matter.

I saw all kinds of birds that I hadn't seen up until that time, and later in the afternoon some chimpanzees gradually got nearer and nearer. We could hear them calling in the distance.

There was an old chimpanzee sitting up in the tree, he had no idea we were there. He sat chewing on leaves and I realised I wouldn't make a very good scientist because after about an hour I became incredibly bored. And when he saw me fidget, he grimaced as if he had just bitten into something truly revolting, and he stood up and took a piss at me. That makes you feel unwanted, for a start.

Then he made a terrific noise followed by short screams, they really were effective screams, and this brought down all the other males in the troop, and I suddenly realised chimpanzees look very big as they come towards you. They weren't behaving as you imagine chimps should - they ought to flee, and they were slapping the trees with their hands, it was a tremendous noise . . .

And I suddenly realised the sound of mokele mbenbe was actually the sound of chimpanzees whooping across open water, this big open space is the only space the natives have any experience of and they are terrified and confused by echo and reverberation.

So the call of mokele mbenbe is chimps whooping it up. I decided that was my one contribution to science to date.

Redmond O'Hanlon is writing a book about this journey.

(Photograph omitted)