This vast river of frozen pinnacles and wafer-thin plates of ice inches its way down from the Continental icecap for more than 18 miles between the steep mountains of the pre-cordillera of the Andes, in the far south of Patagonia. Perched on the edge of the lake from whose flat surface the sheer wall of the jagged glacial mass rises, my hotel bedroom afforded me a stunning view of it on my first morning.
The night before had been a long one. A drive from east to west across Patagonia had taken up most of it. Merely getting to the glacier prepares you a bit for its magic. Patagonia is another world.
Eduardo the taxi-driver and I crossed the plains for hours on end. He drove, I slept. And whenever I woke, the view from the car window was the same: a long, straight ribbon of road stretching endlessly into the distance across the windswept plateau. It was mesmerising driving and Eduardo needed a break, so he got out for a smoke. I walked outside in the cold air. Hanging over me, a vast black dome reached from horizon to horizon, as resplendent with stars as the road was empty of cars.
Driving on through this immense, barren land, nothing dimmed the dazzling display of unfamiliar constellations. Unfamiliar, too, as we neared El Calafate, were the strange shapes of the trees, lit up by the lights of the car and stretching away on either side of the road in mysterious woods of Tolkien-like country.
The drive had prepared me for surprises, but the first sight of the glacier took my breath away. Was it possible that ordinary mortals could actually walk on it? "Yes, claro," Jaime, the guide, reassured me. "I'll teach you to walk with crampons." I glanced around at the rest of the small group of adventurers gathered at the edge of Lago Argentino that frosty morning. Did they look as if they knew how to cope with crampons?
A motley selection of anoraks, jeans and woolly hats, adorning a variety of shapes and sizes of all ages, reassured me. A few youthful bodies would no doubt have a slight edge on me in the fitness stakes, but I could always puff along at the rear.
The wind tore at our thick clothing and whipped up the milky aquamarine water of the lake as we crossed in a small boat to the rim of the glacier. Above its roar, Jaime explained the process that makes the Perito Moreno glacier unique. The tip of the glacier slowly grows until it reaches the far shore. As it advances... "How quickly?" I asked. "Oh, about one and a half metres every day. Till it forms a dam in this narrow channel." He pointed to it on a map. "As the level of the water rises, so the pressure also rises. Then ...."
The following apocalyptic explosion draws people from all over the world to watch monoliths of ice, many the size of a block of flats, break off the glacier with a thunderous roar. The last break-up was in 1988; weren't we due for another?
It seemed likely to happen that very hour, as gunshot explosions of cracking ice broke the silence. The only other sound was the crunch of our plodding footsteps on the crispy snow that covered the glacier. Jaime had duly explained the secrets of walking with crampons. "Keep your knees bent, feet quite wide apart, back straight. It will feel strange in the beginning."
It did, but it worked. Inching my heavy-footed way along ridges, through caves of blue ice and round strange-shaped pinnacles, worn over centuries by wind and weather, I came heart-stoppingly close to crevasses. What if I fell into one of those narrow, bottomless slits of ice? "Oh, we have all the equipment. We also have a Bible."
The Bible wasn't needed; and fear and exercise had given me a good appetite for lunch. Crampons off, we strolled beside the lake under the midday sun. We ate our picnic sitting on rocks that one day will again be covered by the waters of the lake.
"You should see the glacier from the Magellan peninsula," said Jaime. "You get a better idea of its size from there." So in the early evening I walked through the sub-antarctic woods to reach the wooden catwalk that fronts the two-mile long headwall across a narrow channel. Jaime was right. Rising to a height of 180 ft above the level of the lake, the glacier was a moving, living entity; a monster that groaned and creaked as it advanced.
As I faced this frozen colossus, I was suddenly aware that I was watching the face of the earth changing. New contours of the earth were being forged, millimetre by millimetre. In aeons to come, what is now a ferocious leviathan of ice will surely one day be a gentle U-shaped valley, alive with wild flowers and the soft lowing of cows.
By the end of September, discount travel agents will sell return tickets on British Airways from Gatwick to Buenos Aires for around pounds 525