`We used to come up that quite a lot,' murmurs Don, leaning forward to point at it.

I look at him in astonishment and then back at the terrifying path. We are hovering now just feet from it and the dull thudding of the rotor blades is reverberating back at us. The pathway is just a foot or two wide, grassy and slippery.

Yes, I suppose it is a bit steep,' says Don with a gentle laugh, as if that was the only reason they didn't do it by bicycle. `There's a track and bowl system up on top of that ridge ahead of us. Want to take a look?

We nod nervously and Bill flies on.

I had heard the term `track and bowl system' bandied about by New Zealand zoologists before, and they had bandied it about so casually that I hadn't immediately liked to say that I hadn't the faintest idea what they were talking about. I decided to start from the premise that it was something to do with satellite dishes and work it out gradually from there. This led to me being in a state of complete incomprehension for about two days before I finally plucked up the courage to admit my ignorance.

A track and bowl system is nothing whatever to do with satellite dishes. It does, however, share with them this feature - that it is likely to be found in high, open places. It's a rather odd name for an extremely odd phenomenon. A track and bowl system doesn't look particularly dramatic, and indeed if you were not a New Zealand zoologist you might pass one by without even noticing it, but it is the site of one of the most peculiar pieces of behaviour performed by any animal on earth.

The helicopter sweeps out beyond the ridge into the open valley, turns and approaches the ridge again from the other side, lifts on the updraught, turns slightly again - and settles. We have landed

We sit in stunned silence for a moment, scarcely believing what we have just landed on. The ridge is only a few yards wide.

It plunges for hundreds of feet on either side, and falls away rapidly in front of us as well.

Bill turns and grins at us. 'No worries,' he says, which I thought they only said in Australia. This is the kind of thought you need to distract you at moments like this.

Nervously we climb out and, tucking our heads under the turning blades, scramble out on to the ridge. Spread out around our promontory is a deep jagged valley plunging away from us on three sides, softening in its contours at its lower levels. Just beyond us it makes a sharp left turn and proceeds by a series of sharp twists and folds to the Tasman Sea, which is a hazy glimmer in the far distance. The few clouds, which are not that far above us, trace the undulations of the valley with their crisp shadows as they make their way slowly along it, and this alone gives us a clear sense of scale and perspective.

When the thudding blades of the helicopter are finally still the spacious murmur of the valley gradually rises to fill the silence: the low thunder of cataracts, the distant hiss of the sea, the rustling of the breeze in the scrubby grass, the keas explaining who they are to each other. There is one sound, however, that we know we are not going to hear - not just because we have arrived at the wrong time of day, but because we have arrived in the wrong year. There will not be any more right years.

Until 1987 Fiordland was the home of one of the strangest, most unearthly sounds in the world For thousands of years, in the right season, the sound could be heard after nightfall throughout these wild peaks and valleys.

It was like a heartbeat: a deep powerful throb that echoed through the dark ravines. It was so deep that some people will tell you that they felt it stirring in their gut before they could discern the actual sound, a sort of wump, a heavy wobble of air. Most people have never heard it at all, or ever will again. It was the sound of the kakapo, the old night parrot of New Zealand, sitting high on a rocky promontory and calling for a mate.

Of all the creatures we were searching for this year it was probably the strangest and most intriguing, and also one of the rarest and most hard to find. Once, before New Zealand was inhabited by humans, there were hundreds of thousands of kakapos. Then there were thousands, then hundreds. Then there were just forty... and counting. Here in Fiordland, which for many thousands of years was the bird's main stronghold, there are now thought to be none left at all.

Don Merton knows more about these birds than anyone else in the world, and he has come along with us partly as our guide, but also because this flight into Fiordland gives him the opportunity to check one more time: has the last kakapo definitely gone?

Our helicopter is perched at such a dizzying angle on the high ridge of rock it looks as if the merest puff of wind will toss it lightly away into the valley far below us. Mark and I walk slowly away from it with a stiff, uneasy gait as if we are aching all over. Any move we make we make first with our heads before daring to move the rest of our bodies. Bill Black grins at, us wickedly for being earthbound city boys.

`No worries,' he says cheerfully. `Wherever we can land we put down. This is where Don wanted to come so this is where I put him. Wouldn't want to be here if there was a high wind blowing, but there isn't.' He sits on a small rock and lights a cigarette. `Not right now, anyhow,' he adds and peers off into the distance, happily contemplating the enormous fun we would all have if a gale suddenly whipped up along the valley.

Gaynor feels for the moment disinclined to move too far away. from the chopper, and decides that this might be a good moment to interview Bill. She pulls the tangled coloured cables of the cassette recorder out of her shoulder bag and jams the tiny headphones over her hair, without ever looking down to the left or the right. She thrusts the microphone at him and uses her other hand to steady herself nervously against the ground.

`I've been flying in Fiordland for fifteen years,' says Bill, when she's ready, 'mostly telecommunications work, and some construction work. Don't do tourists usually. Can't be bothered with that. Otherwise I do a lot of work for the kakapo transfer programme, flying the wardens around to the most inaccessible parts of New Zealand. A helicopter's very useful for that, because it can put down in the most unlikely places. You see that rocky peak over there?

`No!' says Gaynor, still staring fixedly at the ground. `I don't want to look yet. Just... tell me a story. Tell me . .. tell me something funny that's happened to you. Please??

'Something funny, eh? says Bill, and takes a long thoughtful drag on his cigarette as he surveys the valley. `Well, I once set my hands on fire in the helicopter, because I lit a match without realising my gloves were soaked in petrol. That the sort of thing you had in mind?

Don Merton in the meantime has calmly walked off a few yards, and is peering anxiously at a patch of the scrubby ground. He squats down, and very carefully brushes aside pieces of loose earth and grass from a shallow depression in the earth. He finds something and picks it up. It is small, roughly oval in shape and pale in colour. He examines it carefully for a while and his shoulders sag dejectedly. He beckons us over to join him. We follow nervously and look at the thing he is holding up between his fingers and regarding with extraordinary sadness. It is a single, slightly elderly, sweet potato. I hardly know what to say.

With a sigh he replaces the sweet potato on the ground.

'We call this place Kakapo Castle,' he says, looking up and squinting at us in the cold, bright sunlight. 'It is the last known kakapo booming site in the whole of mainland New Zealand. This shallow pit in the earth here is part of a track and bowl system.'

I'll explain what a track and bowl system actually is in a moment. All there is to see here is the roughly dug shallow pit in the ground. It's untidy and a little overgrown. Looking round again at the breathtaking landscape spread out around us I feel bewildered. We have flown so far into this shattering immensity of land, and all to find these small sad scrapings in the earth and no egg, just a potato.