Africa's biannual miracle is being performed again. The largest and longest overland migration on Earth is in full spate. More than two million wildebeest and zebra, with a few thousand gazelle hanging around the fringes, have reached the south-east corner of the Serengeti, in the early stages of their 800km northward trek from the parched plains of southern Tanzania to the damp grasslands on either side of the Kenyan border.
After a few days' grazing to restore their strength, this impossibly huge, braying assembly will risk life and limb by resuming a journey that they and their ancestors have been making twice a year since prehistory. Defying the long odds against their survival, they will set a course far beyond the horizon, following the smell of rain on the wind. Nine out of 10 will reach their destination but, just as they become accustomed to the creature comforts of their new home, they will up sticks again and head back to where they started. It almost defies belief.
It's said that wildebeest can sense moisture more than 100km away, while their striped accomplice, the zebra, is renowned for its keen eyesight. Together, this unlikely team - large and little, galumphing and dainty - has a greater chance of negotiating the obstacles in its path. The two unrelated species complement each other perfectly at mealtimes. The zebras out ahead, partial to the long grass, take the first course, trimming it neatly for their friends at the rear, who like it nice and short. But lurking along the way, among clumps of trees and rocks, or in the wide, sharp-stoned Grumeti River that forms a barrier between one feeding ground and the other, is all manner of danger. A tiring, disorientated member of the column, separated from its family or group, is easy prey for the big cats, who spend most of their lives in utter indolence, secure in the knowledge that the next meal will present itself soon enough.
Injury plays a part in depleting the convoy's numbers - a wildebeest or zebra which snaps a leg on the rutted terrain is left to die or be eaten. Young and old collapse from exhaustion, and a feathered frenzy of vultures and storks do the rest, leaving only the tough skin and dry bones for the lowest of the scavengers, the hyena. And when the great procession at last crosses the river, at roughly the same place every year, circling crocodiles are lying in wait.
For the human observer on safari, this is a potentially epic spectacle, but although the essential patterns are repeated from one migration to the next, they don't happen like clockwork. Tanzania's drought ended late this year, and the wildebeest are about a fortnight behind schedule.Furthermore, the animals seldom proceed in a straight line. This can make them fiendishly difficult to track.
Nothing prepares you for the sheer immensity of the Serengeti National Park, the largest in Africa. The English language lacks the words to describe the open, flat, sharply-lit vistas of grass, rock and restless skies that stretch away in every direction.
For the 2006 season, the safari and conservation specialist, CC Africa, has developed a new style of mobile camp that virtually guarantees a successful interception in any month of the year. Tanzania Under Canvas moves fully-serviced, luxurious tents - six at a time - from one designated site to another, observing the migration in close-up. And this being the Serengeti, an exciting array of permanently resident animals and birds are also on daily display.
Last weekend a light aircraft dropped me off at Seronera airstrip, little more than an arrivals hut and a windsock, but for the next few weeks the principal gateway to the migration. A few minutes down the road my driver/guide, Mohamed, pulled up about 20 metres from an unremarkable-looking date-tree. "Look closely," he said. All I could see were leaves and branches. "Look again." It took a few more seconds for my untrained eye to pick out a large leopard among the foliage, rousing itself from sleep, surveying the tall grass for a possible kill. I had bagged the first of the Big Five - less than 10 minutes after arriving in the Serengeti. Considering the mobile camps can be dismantled, packed and reassembled in a new location within three days, they are astonishingly well-equipped. Last week the location was Rongai Number Three, a wooded hill with a panoramic view of the plain and the distant Ngorongoro mountain. Camouflaged among the trees were six double-tents for guests, each with an awning for sitting out and an annexe for private dining, a large mess tent for communal eating, and a huddle of canvas for the 12-strong staff.
Being about 1,700 metres above sea level, the evenings can be shiveringly cold, so a hot-water bottle is essential. Meals are served on white tablecloths with crystal wine glasses, and the evening begins, after the rapid equatorial sundown, with pre-dinner drinks around a roaring log fire which helps to deter unwelcome four-legged intruders. But the best amenity of all is natural: the unforgettable sight as you zip back the tent-flaps at dawn and the Serengeti is laid out before you like a giant watercolour.
On the morning drive, it takes Mohamed an hour to locate the migrating horde. Individually, the animals may be unremarkable, but collectively they are phenomenal. They have leaders who decide on the best route to take. Some form of daily conclave takes place, assessing the grazing conditions and sources of danger. The zebras are posted as look-outs. At ground level, the grouping appears to be random, but from the vantage point of Naabi Hill, their spacing and formation can be seen to have an almost military regimentation. One day soon, they will suddenly get up and go, covering 30-40km without a break. This may be sufficient to shake off their predators, but the two-legged guides from the mobile camps will doggedly remain on their trail.
The writer travelled with Rainbow Tours (020-7226 1004; www.rainbowtours.co.uk) which offers seven-day safaris from £2,675 per person. This includes return flights to Dar Es Salaam, transfers, full board at Ngorongoro Crater Lodge and with Tanzania Under Canvas, game activities and national park fees.
The environmental cost of a return flight from London to Dar Es Salaam is £14, available through Climate Care (01865 207 000; www.climatecare.org).
British tourists require a visa (£38): 020-7569 1487; www.tanzania-online.gov.uk.