Crappy magnetic rocks in a box. This was my gift-shop-tourism takeaway the last time I stood at the Grand Canyon's edge. Those smooth pseudo stones were meant to pass for a connection with the inner workings of America's greatest natural wonder. And, as an 11-year-old boy, I really tried to believe that. Perhaps no one who merely stands at the edge of the abyss can hope for any more. Yet, as I was harried between the retail experiences and tripod-wielding tourists of the ever-busy South Rim, I like to think that I made a promise. A cross-your-heart-and-hope-to-die pact, that one day I would get all the way to the bottom of that big hole and do some real exploring.

Fast forward two decades and I'm speeding away from the scene of that promise through an Arizona November night. My Californian travel companion James and I left the South Rim at 4am, heading to the remote North Rim of the Grand Canyon. We will attempt the 'Rim to Rim' hike, taking us down into the Canyon, along its hidden flats and back out to our starting point. It's only 23 miles, but to reach the trailhead you must first drive the 212-mile overland route around the great rift in
the Earth.

At the wheel is our driver Marcia, and she's just finishing her pep talk: "Remember, be careful near the edge. It's soft. The hard rock is at the bottom of the canyon," she says deadpanning the Brit in the rear-view mirror, "so it'll hurt a lot if you fall." Jokes aside, it's the cold that I'm really worried about. After four hours' driving we reach the North Rim Kaibab Plateau, where the mercury reaches a low of -6°C. At 2,500m we are nearly twice the altitude of Ben Nevis. Dawn reveals a sparkling landscape of ponderosa pine, frosted meadows, and some horsey looking remains being dissected by a feathered velociraptor. "Golden eagle," says Marcia, cooly.

We spend an hour at Bright Angel Point, watching as sunlight chases shadows into the darkest basement of the Canyon floor. It's church-like quiet. Looking across the great trench at the journey ahead, successive ridges and rock spurs appear from blue-morning haze like layers of illuminated theatre scenery. On the very far side is South Rim, where we hope to finish before the day is out. It's time to get moving.

After hugs with Marcia, we begin. The North Kaibab Trail immediately plunges through the tree line, and once off the plateau the air temperature is far more agreeable. Mountain ash and gambel oak cling hopelessly to scant November leaves, and at each hairpin we have an unobstructed view of the fall-line to what we presume, incorrectly, is the Canyon's basement.

And yet the expected floor doesn't come. The convex architecture of the Canyon tempts you down through its many false bottoms. Disarming on the way down. Devastating on the way back up. Instead, we step through the narrow rock-blasted Supai Tunnel and emerge into a lower red underworld. This ancient rock band (think less Rolling Stones, more Hermit Shale) sends us on an even steeper descent.

The Colorado River passes under our feet in a rush of darkness and spray

Down here we are entering what early Colorado River explorer John Wesley Powell called "deep time." For every metre of descent we go a million years further back in geological history. The trail narrows to a rocky parapet that hugs the canyon wall, teetering between acres of sheet vertical rock. On the other wall, ancient river eddies are remembered with hot-air balloon-size hollows carved into the Canyon's polished wall.

"Welcome to Texas," James wise-cracks as we pass Cottonwood Campground and stride out into the surprisingly broad cactus-peppered plains of the lower canyon. The previous day we had read in the visitor's centre how the change in flora and fauna while descending from the Grand Canyon's cool high-altitude rim to its desert-like depths is the same as if travelling from Canada to Mexico. Seven more gently descending miles now separate us from the Colorado River.

The temperature is in the pleasant 20s and we are glad to be hiking in balmy November warmth, avoiding summer hiker's heat exhaustion, when temperatures can reach 49°C. As the afternoon rolls on, we concentrate on restoring energy levels by noshing on some nasty cereal bars. "Mexico is close now," I quip lamely back. "What's the chance of a michelada and a burrito at the Colorado River?"

We reach the outlying cabins of long-anticipated Phantom Ranch just as evening light turns grapefruit-pink. A mule deer dines casually alongside the path and seems not to notice as we patter wearily past. Moments later, a stone-built lodge appears through the trees with two dozen hikers sloshing back wine and a mean-looking meal inside. The electric-glow of civilization at the very bottom of the canyon makes us wish we were more organised. While reservations here need to be made 12 months in advance, eating well, breaking up the hike and spending more time in the Canyon is quite clearly the smart choice.

Instead, we check and then re-check our map, which stubbornly continues to indicate a 9.5-mile hike up to our motel near the South Rim. As tipsy Americans stumble sleepily through the dusk to their cabins, stars pop out through a cobalt-clear sky. The temperature drops. I pull on a fleece, refill my water bottle and we push on.

The Colorado River passes under our feet in a rush of darkness and spray. James, who was so chirpy up until now, freezes in the middle of the dark suspension bridge. "What's up, scaredy cat?" Following the beam of his headtorch, the answer is rising behind a cloud-scudded rock escarpment. As a full-moon floods the Canyon we switch off our redundant lights and, once on the Bright Angel Trail, turn to walk alongside the Colorado where frothy whitecaps now sparkle with momentary moonlight.

Many uphill hours follow. Early on in the climb, a furry tail slinks down a tree and into the undergrowth. "Ring-tailed cat?" I venture. "More like mountain lion" teases the Californian, getting me back for my childish bullying on the bridge. Luckily, Mufasa doesn't make another appearance during the endless switchbacks and slowly we make our way back to the Earth's surface.

Over the last few miles we curse the sleeping campers in juniper-scented Indian Garden Campground and start to despise the Canyon's cruel convex curve that provides a never-ending rock wall to our journey. It's a little before 1am when we finally reemerge at South Rim.

On the sleepy drive back to our motel we stop briefly at the viewpoint that I visited as a child. It's silent and empty in the moonlight. Nobody pushes me around, and I'm tall enough now to see well over the guardrail. For a long time I gaze with new familiarity over the secret folds of my childhood imagination. If nothing else, I reason, at least those crappy magnets were enough to attract me back.

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