Two years ago, I had an unnerving experience while winter mountaineering in the Cairngorms. Having woken at a bothy in Glen Feshie to a clear, frosted morning, we climbed above snowline in the watery sunlight to begin what should have been a simple loop—summiting the nearest Munro before following a track north beyond a steep-sided corrie and dropping down to where we’d left the car.

As we approached the peak a bank of snow clouds appeared in the east, moving fast, and soon we were engulfed in the most complete white out I’d ever experienced. The ground, the sky, the boundary between them, all completely obscured and aglow with a flat, white light. The track nowhere to be seen. With visibility down to zero, we bunched together and trudged onwards through a featureless haze, eyes on the compass and counting our steps.

It was dizzying, disorientating. My eyes, searching for a hold on this otherworldly scene, grasped at what reference points it could: cliffs reared suddenly out of nothingness and raced forwards only to transform, abruptly, into thin strands of frozen grass, poking through snow. It soon became impossible to tell whether we were travelling uphill, or down.

Under normal conditions, human perception works so well as to render its workings invisible to us. But in certain circumstances—extreme weather conditions or extraordinary places—we push beyond its limits, sliding into a world of illusions as our brain struggles to make sense of its surroundings.

Depth perception, depending as it does on a number of visual clues, is first to go. In flat light conditions in the Canadian north, 19th-century explorer Otto Sverdrup and his party once spotted a distant herd of caribou. On closer inspection, it transpired to be a group of rabbits.

Polar explorers were well aware of the lies their eyes could tell them. As Frederick Cook wrote of the Arctic in 1911: “Mirages turned things topsy-turvy. Invented lands and queer objects ever rose and fell, shrouded in mystery.”

The invented lands he speaks of are the result of “looming,” or superior mirages. Formed when temperature gradients between strata of air create mirror-like surfaces, distant objects—far beyond the normal horizon—are reflected and produce in the human mind an impression of mountain where there is only sheet ice, or island where there is only sea.

A century earlier, in 1818, the British explorer John Ross spied a handsome range blocking his intended path through the Northwest Passage. He took the time to name them—the “Croker Mountains”—then turned back. A year later, his lieutenant returned and sailed straight through them.

Pushed to physical extremes and unable to parse the ethereal landscape, explorers reported disquieting phenomena—not least psychological ones, like that noted by Ernest Shackleton after a gruelling trek across South Georgia. “At the extremity of their strength,” TS Eliot summarised later, “the party of explorers… had the constant delusion that there was one more member than could actually be counted.”

Such travellers are far hardier than I, of course. Back in the Cairngorms—after falling through a snow arch into a river—we made the decision to turn back. Plodding back the way we came, we found our footprints already filling in. After 40 minutes we reached the point where we expected a route down, and found nothing. I began to fear we’d made a fatal mistake.

And then—what luck—the clouds parted and a shaft of sunlight illuminated the ridgeline we sought. It appeared to hover in mid-air, 100 metres ahead, while crags tumbled away to our right. A second later it had gone behind the cloud again, but we were anchored once more in the real world. Safe.

We were very lucky. That same week, an experienced hiker vanished into the blizzard 10 miles to our east. Two months passed before they found the body. And so, though I’ll never forget those hours spent floating in white space, it’s not an experience I’m anxious to repeat.