First they must produce a recent doctor's certificate, as the only way to bring in medical aid is by helicopter from Windhoek, the capital, 700km away. And they must carry with them all their equipment, from tents to food and cooking utensils.Wanting more comfort, we opted for a newly opened alternative, the camp at the Canyon Nature Reserve, which has been set up by Lex Van Den Bosch, a Dutch biologist and tour guide, and his partner Loes Bolle. Now 100 miles long and up to 16 miles wide, this natural wonder is second only to the Grand Canyon as a gouge in the globe.Every year the Ministry of Environment and Tourism allows a limited number of hikers to spend five days walking 53 miles of the canyon's meanders - from the Main View Point opposite Hell's Corner to the hot springs at Ai Ais. Then some 500 million years ago, a fault opened up among them. Widened by glaciation and altered by more faults and wind erosion, canyons within canyons were formed until 50 million years ago a river began to flow down them.

In the mists of geological time, a sea bed was lifted miles above the level of the ocean and weathered into ranges of table mountains. At ground level - amongst sand and quartzite boulders - grew fat succulents, southern Africa's answer to the cactus.Some 110 miles from the town of Keetmanshoop, invisible until you are almost on top of it, the Fish River Canyon is a huge tear in the face of the earth. At regular intervals, interspersed with the huge nests of sociable weaver birds, we passed pale chanting goshawks on the telegraph poles, watching for prey. But we were delighted by the unseasonal storm for, though the Fish River itself was no longer running when we arrived, water was still cascading down the creeks and seeping through the Canyon floor to replenish the permanent pools. We drove down to base camp across mile after mile of high desert and scrub savannah. Worried and apologetic because of the state of the rough lane, the manager of the Canyon Nature Park rang to warn us that the camp could only be reached in a 4x4. Over two-and-a-half days, 74mm of rain fell in the Fish River Canyon - a whole year's rainfall. The gravel road was covered with treacherous drifts of sand deposited by ephemeral rivers; part of the lane leading to the canyon itself was washed away and the airstrip was filled with fissures created by swift flowing floods.