Just 10cm of glass separates me and a 1.3km drop to the jagged rocks of the Grand Canyon's floor. If I fell from here, I'd have a long time to ponder my life before I was splattered very thinly over a wide area.

I'm on the Skywalk, the horseshoe-shaped, glass-bottomed viewing platform that juts out 22m over the Grand Canyon's precipitous West Rim. Closer to Las Vegas than the more-visited South Rim, it's only a 35-minute flight (or a three-hour drive) from Sin City.

Yet the contrast between the planet's most breathtaking man-made attraction, and one crafted by nature, couldn't be more extreme. Vegas is a party city to the power of 10, a place fuelled by gambling, booze and flesh.

The Grand Canyon, on the other hand, is an adrenaline rush of a different sort. Nothing can prepare you for the first glimpse of this gash in the Earth's crust that is so vast, it's visible from space.

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Perhaps it's the sheer scale that makes jaws drop: stretching 446km long, the canyon wiggles between 6km and 28km across and, at its deepest, is more than 1.6km from top to bottom. In a nation where size matters, they don't come much bigger than this.

We prepare to join the five million other tourists who make the pilgrimage to the Canyon every year, with a 7am pick-up from our Vegas hotel. Even at that hour, there are punters propping up slot machines, being served free alcohol by bored waitresses in fishnets.

Our flight is from Boulder City, created in 1932 for the men who built the Hoover Dam. We fly over the dam for a bird's-eye view of this engineering triumph that remains one of the world's largest concrete structures.

At the West Rim airport we're met by Wilfred Whatonome. An elder of the Hualapai (pronounced "wool-a-pie") Tribe that owns more than a million acres of land bordering the Grand Canyon, Wilfred is rightly proud of the US$30 million (NZ$39 million) architectural wonder that's brought about half a million visitors to his reservation since it opened a year ago.

"This is a unique, sacred place and it's our pleasure to share it with the world," he tells me as we jump onto the bus.

We alight at Eagle Point - so called because the triangular dip in the canyon's ridge looks like a bird with outstretched wings - for the Skywalk. Astonishingly, there are no barriers or warning signs at the canyon's edge. But then how do you fence something so vast?

Apparently one in every million visitors each year fails to use the return portion of their tickets, having fallen over the edge or, for those who venture into the canyon, died of thirst or heat exhaustion. Wilfred reassures us there hasn't yet been a fatality at the West Rim.

After leaving our bags in lockers (cameras and cellphones aren't allowed on the platform in case they fall and mark the tempered glass floor), we're screened by metal detectors and don cloth booties, which help prevent slipping.

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I've read the safety information: kick-ass girders are anchored in place by 108 giant steel bolts driven 12m into the bedrock; the platform could support the weight of 71 fully-loaded Boeing 747s; giant shock absorbers keep it from moving, and rigorous safety checks have been conducted.

But knowing something and believing it are two different things.

The first steps out into the void are electrifying: Wilfred tells us not to look down until we've got our bearings, so I fix my eyes on a spot on the horizon. When I feel it's safe, I cast my eyes down to see the ground melt away beneath my feet. It is, literally, like walking on air. Nowhere else in the world can you walk straight off a cliff and not end up in a million little pieces.

Wilfred calls it a sheer, natural high; I call it one of the most exhilarating things I've ever done. I'm amazed how quickly I get used to the sensation and end up lying face down on the glass, gazing at the rust-coloured valley floor far below.

I've wanted to visit the Grand Canyon ever since I saw it on an episode of The Brady Bunch. I'm happy to report that both it, and the Skywalk, were astounding - like meeting one of your childhood heroes and discovering that they were far cooler than anything you could have hoped for.

Sharon Stephenson was a guest of Air New Zealand and Las Vegas Tourism.