TANGA, Tanzania

After leaving Zanzibar Town, we made a short hop up the island to a hidden cove that is a little asterisk in the long and bloody slave trading history of East Africa.

Up until the end of the 19th century, Arab trading caravans penetrated deep into the interior to purchase human beings for cloth and beads. The unfortunate slaves were then forced to carry heavy burdens of ivory hundreds of kilometres to the coast, joined together by chains and heavy wooden yokes around their necks.

If one of them grew sick or weak and was not able to continue, he or she was left to die at the side of the caravan track, which was littered with skeletons. Hyenas and vultures followed the parade of human misery and thrived.

After reaching the coast, slaves were sent to Zanzibar by ship. They would be chained together in a sitting position, knees to chin, one slave directly behind the other, packed in rows so tightly that none of them could move. Then a floor was installed inches above their heads so another layer could be loaded directly on top, a thought that makes me weak with claustrophobia when I try to imagine it.

Ships might take several days to reach Zanzibar, during which time the slaves would not be able to move, nor were they given food or water. A significant percentage of them died on the voyage, but the dead would stay linked to the living, who remained pressed against their putrefying bodies until the horrible voyage was over.

Once having carried human cargo, a slave ship was fit for no other purpose. The overwhelming human filth, death and decay created a stench that could never be eradicated. But human lives were cheap, and if slave traders ended up profiting from half the slaves they had originally purchased, they were satisfied.

After the British finally convinced the Sultan of Zanzibar to officially abolish the slave trade in 1873, the trade continued, more secretly, for some time. In the little cove where we were now anchored, slavers could quickly tuck in if a British patrol ship was in the neighbourhood and march their slaves to a hidden cavern. Here they could be concealed until the threat of discovery was passed.

From the standpoint of the slaves, being stacked like firewood in an underground cavern was better than the alternative, because slaving ships on the verge of being caught usually just dumped their human cargo, still joined together and weighed down by chains, into the sea.

We headed ashore to find the path to the slave caves, on the way passing through plantations growing cassava, bananas, coconuts and tomatoes. At the end of a tiny overgrown track we found it: an underground bunker built from stone, mangrove tree sticks, and mud. The roof was now mainly gone, and in a few years the whole structure will collapse, but we could see how hundreds of people would have been herded down into the dank, dark chamber. There was provision for a removable floor so a second layer of slaves could be herded in directly over the heads of the first. From a few metres away, the overgrown, half buried structure was unnoticeable, and from the nearby beach it was invisible.

The next day we continued north to the northern tip of Unguja Island, which is the proper name for what is usually known as the Island of Zanzibar. In fact, Zanzibar comprises the two neighbouring islands of Unguja and Pemba. Once upon a time, the Sultan of Zanzibar and Oman controlled virtually all of East Africa. But gradually the colonial powers whittled away at his empire, and in 1964 the Zanzibaris overthrew the last Sultan in a bloody revolution in which 17,000 Arabs and Indians were massacred in a single night.

Under Marxist rule, Zanzibar closed itself to the outside world, only re-emerging in the past decade or so. It has really only been in the past five years that tourists have begun to venture here in any numbers. A few years after the revolution, Zanzibar joined up with its mainland neighbour of Tanganyika, forming the present country of Tanzania. Yet in many ways, Zanzibar still considers itself its own nation.

Continuing north, we anchored at a calm bay in a middle of a fleet of wooden fishing dhows, many with gaping holes in their old cotton sails. It had been weeks now since we had seen a single other cruising sailboat.

We continued to Ras Nungwi, the northernmost tip of Unguja Island, where we took a long walk through a series of primitive little villages. The women of northern Zanzibar are covered and robed as modestly as are the ladies of Stonetown, but here, instead of black, they wear beautiful patterned and multi-coloured robes and scarves of Swahili rather than Arabic origin.

The visual effect of these colourful swathed ladies against the uniform grey of their cobbled coral-stone houses was stunning, but to our disappointment no one wanted to be photographed. If we tried, they would wave us away, turn their backs, or even run into their homes. Our reception as we walked through the village was much friendlier as soon as our cameras were safely stowed away.

The next day we had a bumpy sail across the channel to Pemba Island. We spent most of the morning checking in with various authorities, receiving both inward and outward clearance for Pemba, even though we were already officially cleared into the country and were continuing on to another Tanzanian port. The officials here like to have the chance to inspect our papers, make us fill out useless forms and, if possible, extract a little money from us each time we stop.

As I waited demurely outside with the children as a good wife should, I could hear Herbert good-naturedly bantering with officials who wanted to charge him various unnecessary port dues or come back again later. Then we had to look hard to find the immigration official, whose office was hidden in the back of what looked like an abandoned and condemned building with a falling-down ceiling and broken windows. As we stood there in an empty outer room, a door opened and a woman with a young child peered out suspiciously at us, then nervously banged it closed again.

Eventually, the uniformed official arrived and brought us into his dark, dirty and decrepit office where we filled out more forms, to be added, no doubt, to the mouldering piles of filthy papers already stacked on the floor. Then he stamped our passports for our departure to the Tanzanian mainland the next morning, clearing us out of Zanzibar exactly as if we were heading off to a foreign port. It has been a universal rule, as we have travelled around the world, that the smaller and poorer the country, the more paperwork and officials are imposed upon us.

The town of Mkoani was small, dusty and poor and had nothing in it of interest as far as we could see. We saw several boys aged 10 or 12 pushing around little homemade wooden trucks on the end of a stick. The ragged children here really had nothing, and these pitiful little toys made Herbert and I cringe, although our children didn't have the same reaction, admiring their counterparts' resourcefulness.

While buying a bunch of delicious mandarin oranges for pennies apiece, two little girls of perhaps four and six years of age stood by and inspected us. Although they were carrying their own plastic bag full of mangoes, we felt guilty as we slurped on our juicy oranges in front of them. The wealth represented by the cameras in our backpack was probably more than their family could dream of earning in an entire lifetime. The girls were literally wearing rags, their bare shoulders poking through gaping holes in their dresses. They obviously weren't starving, but we wondered whether we should be sharing our fruit with them. But for some reason we didn't, and even now, when I think of those two little girls, I am ashamed.

We went for a walk out of town, followed like the Pied Piper by a rapidly swelling group of children. Michael found a small toad near the riverbank and scooped it up. When he showed it to the local kids, they all scattered and screamed in terror, boys and girls alike. Pemba is one of the main centres of witchcraft in Africa, with even voodoo specialists from Haiti reportedly travelling here for instruction. I wondered whether the children's fear of the toad might have something to do with these ancient but still powerful beliefs.

Our boys found some sticks and engaged themselves in Star Wars light sabre duels while the Pemba kids stood by and giggled. I enjoyed observing the girls, who unlike most we had seen, were not robed from head to toe and could therefore play freely. They had very short hair on their bare heads, just a little bed of springy black whorls. It was surprisingly attractive, and certainly practical. These little girls, although they wore dresses, were as athletic and gregarious as their brothers and I liked them for that.

I decided to offer a small bag of cookies, which was disappointingly crumbled when I opened it. I held it out and the kids, as soon as they realized what it was, dived into it en masse, about 20 grubby hands reducing it instantly to a pile of brown cookie dust. In seconds even the dust was gone, but there were lots of smiling faces.

A few minutes later, one of the smallest girls came over and shyly presented me with a mango. I usually carry a few little toys with me for occasions such as this, and I rummaged around in my backpack and brought out a bright pink and yellow bouncy ball. In a place where a block of wood with four wheels is a fancy toy, I was confident a Power Rangers ball would be a real prize. Concealing it in my hand so the other kids wouldn't see it, I advanced toward the little girl, who was perhaps six.

She ran away when she saw me coming, but then I opened my hand to reveal what was inside, and her eyes opened wide. She darted eagerly forward and snatched the ball, and then began sprinting down the dirt road. Half a dozen other kids, curious about what I had given her, took off after her and they all ran, screaming and laughing, into the distance.

I hope the little girl got to keep her trophy, but we never saw her again.