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A little blue biplane bumping over the floor of a featureless desert is depicted in a film made to illustrate the capabilities of IMAX - the ultramodern cinema with the five-storey, wrap-around screen. Suddenly, shockingly, the biplane reaches the rim of the Grand Canyon and goes down over the edge. And you go with it, twisting and tumbling into that incredible abyss.
But no film can prepare you for the awesome impact of the canyon itself. Even blase busloads of teenagers are muted by the existence of something so vast, so old, so serene and so silent. This tremendous trench carved by the Colorado River is a dizzying 1 mile (1.6km) deep and 9 miles (15km) wide, on average, and stretches for some 280 miles (450km). At any given 13-mile (21km) stretch, you could drop in the entire island of Manhattan and still need field glasses to inspect the tops of the skyscrapers beneath you.
Its moods and colours change with the hours and the seasons. At dawn, the striped rocks on the opposite rim - far distant yet seemingly close enough to touch - are lacquered silver-old above the blue chasms below. Spring mornings fill the depths with mist that looks solid enough to snowshoe on; moonlight floods the distances and flushes the upper cliffs to the deepest rose.
Most rain evaporates before it reaches the bottom of the chasm, so down beside the river, the terrain is an arid desert. By contrast, the North Rim, which is considerably higher than the South Rim, is covered by Arctic snow until well into May. Between these two extremes, the climate shadings range through tundra, temperature woodland and coniferous forest. As when climbing a mountain, every 1000ft (300m) of elevation produces a climatic change compared to a 300-mile (500km) journey northwards over the flat.
This variation allows a rich range of wildlife - mountain animals such as bighorn sheep become near neighbours of such desert creatures as rattlesnakes. And the canyon's great divide ensures that certain animals never meet - tassel-eared squirrels of the North Rim are quite different subspecies from those of the South Rim.
Years ago, they say, a wandering cowboy peered down into the canyon's fearsome depths and exclaimed; 'Something sure happened here!' Like many a visitor since he could not credit that the canyon was shaped mainly by the ribbon of the river at its base. Only closer inspection reveals that here Colorado can be a raging torrent, bearing with its tens of thousands of tons of sediment a day - 'Too thick to drink, too thin to plough', the old settlers used to complain. The valley's width is largely due to tributaries fed by melting snow, aided by 5 million years of erosion by wind, frost and rain. The deep, central slash is the work of Colorado.
It is reasonable to assume, as did the early explorers, that the river, over aeons, simply burrowed its way down to its present depth. The river stands more or less where it always did, at about 2000ft (600m) above sea level. It was the land that gradually rose, but so slowly that the scouring power of Colorado enabled it to maintain its original place while the walls of the chasm climbed high on either bank.
The canyon's story is written in the differing ages of rocks as they emerge layer upon layer from the depts. Lowest of all is the inner Gorge, where the colorado runs. Two billion years ago, the dark rock here was part of a mountain range as high as the Himalayas. The mountains were worn down a plain and replaced by a shallow sea. Limestone deposited on the sea floor contains the fossils of plankton that are 1 billion years old.
Upwards, the diary of the rocks tells of ancient cataclysms in which mountains were worn away to be drowned by muddy flood plains where amphibians and reptiles left their footprints. Some layers were once marshes and deserts and further ancient seas in which corals lived. The river did not begin its scouring until 6 million years ago. Yet when lava spread down one end of the gorge less than a million years ago, the canyon was already within 50ft (15m) of its present depth.
Visitors who study the layered pages of this diary of Earth may find the subject matter too enormous to comprehend. Most will feel an awed glow at seeing its secrets revealed.
