The world of smells is now virtually closed to modern man. Not that we haven't got a sense of smell - we sniff our food or wine, we occasionally smell a flower, and can usually tell if there's a gas leak, but generally it's all a bit of a blur, and often an irrelevant or bothersome blur at that. When we read that Napoleon wrote to Josephine on one occasion, `Don't wash - I'm coming home,' we are simply bemused and almost think of it as deviant behaviour. We are so used to thinking of sight, closely followed by hearing, as the chief of the senses that we find it hard to visualise (the word itself is a giveaway) a world which declares itself primarily to the sense of smells. It's not a world our mental processors can resolve - or, at least, they are no longer practised in resolving it. For a great many animals, however, smell is the chief of the senses. It tells them what is good to eat and what is not (we go by what the packet tells us and the sell-by date). It guides them towards food that isn't within line of sight (we already know where the shops are). It works at night (we turn on the light). It tells them of the presence and state of mind of other animals (we use language). It also tells them what other animals have been in the vicinity and doing what in the last day or two (we simply don't know, unless they've left a note). Rhinoceroses declare their movements and their territory to other animals by stamping in their faeces, and then leaving smell traces of themselves wherever they walk, which is the sort of note we would not appreciate being left.

When we smell something slightly unexpected, if we can't immediately make sense of it and it isn't particularly bothersome, we simply ignore it, and this is probably equivalent to the rhino's reaction to seeing us. It appeared not to make any particular decision about us, but merely to forget that it had a decision to make. The grass presented it with something infinitely richer and more interesting to its senses, and the animal returned to cropping it.

We crept on closer. Eventually we got to within about twenty-five yards, and Charles signalled us to stop. We were close enough. Quite close enough. We were in fact astoundingly close to it.

The animal measured about six feet high at its shoulders, and sloped down gradually towards its hindquarters and its rear legs, which were chubby with muscle. The sheer immensity of every part of it exercised a fearful magnetism on the mind. When the rhino moved a leg, just slightly, huge muscles moved easily under its heavy skin like Volkswagens parking.

The noise of our cameras seemed to distract it and it looked up again, but not in our direction. It appeared not to know what to think about this, and after a while returned to its grazing.

The light breeze that was blowing towards us began to shift its direction, and we shifted with it, which brought us round more to the front of the rhino. This seemed to us, in our world dominated by vision, to be an odd thing to do, but so long as the rhino could not smell us, it could take or leave what we looked like. It then turned slightly towards us itself, so that we were suddenly crouched in full. view of the beast. It seemed to chew a little more thoughtfully, but for a while paid us no more mind than that. We watched quietly for fully three or four minutes, and even the sound of our cameras ceased to bother the animal. After a few minutes we became a little more careless about noise, and started to talk to each other about our reactions, and now the rhino became a little more restive and uneasy. It stopped grazing, lifted its head and looked at us steadily for about a minute, still uncertain what to do.

Again, I imagine myself, sitting here in my study writing this through the afternoon and gradually realising that a slight smell I had noticed earlier is still there, and beginning to wonder if I should start to look for other clues as to what it could be. I would start to look for something, something I could see: a bottle of something that's fallen over, or something electrical that's overheating. The smell is simply the clue that there's something I should look for.

For the rhino, the sight of us was simply a clue that there was something he should sniff for, and he began to sniff the air more carefully, and to move around in a slow careful arc. At that moment the wind began to move around and gave us away completely. The rhino snapped to attention, turned away from us, and hurtled off across the plain like a nimble young tank.

We had seen our northern white rhinoceros, and it was time to go home.

The next day Charles flew us back across the ostrich skin savannah to Bunia airport where we were due once more to pick up a missionary flight returning to Nairobi . The plane was already there waiting and a representative from the airline assured us, against the evidence of all our previous experience, that there would be no problems, we could go straight to the plane. Then, a few minutes later we were told that we would just have to go quickly to the immigration office. We could leave our bags. We went to the immigration office, where we were told that we should bring our bags. We brought our bags. Expensive looking camera equipment.

We were then confronted by a large Zairois official in a natty blue suit whom we had noticed earlier hanging around watching us take our baggage out of Charles's plane. I had had the feeling then that he was sizing us up for something.

He examined our passports for a goodish long time before acknowledging our presence at all, then at last he looked up at us, and a wide smile crept slowly across his face.

'You entered the country,' he asked, 'at Bukavu?

In fact he said it in French, so we made a bit of a meal of understanding him, which was something that experience had taught us to do. Eventually we admitted that, insofar as we had understood the question, yes, we had entered at Bukavu.

`Then,' he said, quietly, triumphantly, `you must leave from Bukavu.'

He made no move to give us back our passports.

We looked at him blankly.

He explained slowly. Tourists, he said, had to leave the country from the same port by which they had entered. Smile.

We utterly failed to understand what he had said. This was almost true anyway. It was the most preposterous invention. He still held on to our passports. Next to him a young girl was sitting, studiously copying down copious information from other visitors' passports, information that would almost certainly never see the light of day again.

We stood and argued while our plane sat out on the Tarmac `waiting to take off to Nairobi, but the official simply sat and held our passports. We knew it was nonsense. He knew we knew it was nonsense. That was clearly part of the pleasure of it. He smiled at us again, gave us a slow contented shrug, and idly brushed a bit of fluff off the sleeve of the natty blue suit towards the cost of which he clearly expected a major contribution.

On the wall above him, gazing seriously into the middle distance from a battered frame, stood the figure of President Mobuto, resplendent in his leopardskin pillbox hat.