Stretching across Africa like a gigantic wound, the Rift Valley is more than 6,430km (3,987 miles) long and in places up to 1,520m (4,986 ft.) deep. It cuts through the continent from Jordan all the way down to Mozambique and, when viewed from outer space, is apparently the single most identifiable geographic landmark on the face of the planet. Geologists believe that it's the result of violent subterranean forces that literally ripped Earth's crust apart, causing massive chunks of crust to sink between parallel fault lines. In return, molten rock was forced up to the surface in a series of volcanic eruptions. It was here, some 40 million years ago, that Africa began splitting apart along monstrous parallel fault lines -- and the land between slipped down, creating the Rift's distinctive valleylike appearance. This process -- called rifting -- is still underway, evidenced in the active and semi-active volcanoes that simmer and bubble throughout the Rift Valley. When you see a boiling hot spring or hissing geyserlike steam vent, or hear the irritable rumbling of a volcano, it's evidence that Earth is still furiously at work trying to tear Africa apart.

Millions of years of tectonic movement and volcanic activity have not only literally ripped the land apart, but also divided the Rift into two separate branches. The widest and longest of these is the Eastern Rift, which splits Kenya from north to south in a series of fault lines that give spectacular shape to the landscape. In Kenya, this Eastern Rift (sometimes called the Gregory Rift, after the man who first described the Rift in the 1890s) is synonymous with magnificent escarpments, volcanic cones (more than 30 of which are still active or dormant), lava fields, and a smattering of lakes, most of which are soda. This part of the Rift is up to 100km (62 miles) wide and reaches its narrowest point just north of Nairobi, where it's a mere 45km (28 miles) across. The valley floor is at its shallowest near Lake Turkana, in the far north of the country, where there is virtually no distinction between the Great Rift and the surrounding desert. Farther south, though, the valley walls form sheer cliffs rising to 1,800m (5,904 ft.) at Lake Naivasha, the highest lake in the Rift system and one of the few freshwater lakes in the Valley. South of Naivasha, the Rift descends again to 580m (1,902 ft.) at the Tanzanian border.

Boiling, steam-spurting geysers are found throughout the Great Rift. You'll see them dotted around many of the lakes that are, in turn, speckled with the slender, pink-feathered bodies of thousands of flamingoes. If you're brave enough to explore Lake Magadi in the southern part of Kenya's Rift, you'll witness extraordinary, hellishly hot conditions where almost no living thing -- save the specialized avifauna -- can bear to survive. Conditions are much calmer at Lake Nakuru, where the bitter, alkaline-rich waters create a peculiar habitat for specially adapted organisms, such as special algae and small shrimplike creatures, which are the reason greater and lesser flamingoes are attracted to these soda lakes in the first place. Lake Nakuru is now at the center of one of the most popular wildlife preserves in the country, packed with rhino, plains game, and assorted predators.

Of Kenya's Rift Valley lakes, Naivasha and Nakuru are the most accessible and, together with Lake Baringo, make up a pretty decent road circuit manageable in a couple of days out of Nairobi. The eerie, mesmeric Lake Bogoria -- its surface capped by a flotilla of uneasy flamingoes and its slightly apocalyptic shores dotted with steam-spurting hot springs and scalding geysers -- is an easy side trip from Baringo. Lake Magadi lies just south of the capital and is best seen as an outing from the beautiful Shompole lodge, tucked into the spectacular Nguruman Escarpment, which is also within striking distance of Tanzania's Lake Natron.

Lake Turkana, in the far north of the country, also forms part of the Rift Valley system and sits amid one of the most inhospitable regions in East Africa in a tortured desert landscape that's endlessly punished by heat and high winds.

Touring the Great Rift -- If you want to get as much out of this region as possible, you'll be best off arranging a 4X4 with a driver (and perhaps an expert guide as well) through your tour operator. This part of Kenya can be traveled as a round-trip commencing in Nairobi. Spend a night or two at either Lake Naivasha or Lake Nakuru (or both), and finish off with several relaxing days on one of Lake Baringo's remote islands. From there, you can either drive or fly back to Nairobi, or head over the Rift's western escarpment, descending to the Kakamega Forest for a totally different sort of wildlife experience. You'll pay approximately $275 per day for the hire of a vehicle plus driver (including up to 200km/124 miles travel per day) -- all accommodation costs and sightseeing will be extra. Your best bet for seeing this region is Sunworld Safaris (tel. 020/444-5669 or 020/444-5850;; you can contact them in the U.S. through Siggi Hosenfeld (9011 Mira Mesa Blvd. #226, San Diego, CA 92126; tel. 619/254-2096;, or have your Kenyan ground operator make all the arrangements.