The jelly legs kick in around a quarter of the way up. This is fiendishly cruel, and it's only going to get worse. 
I've taken on my fair share of rather terrifying Australian adrenalin experiences in the past. I've swum in a cage with a five-metre crocodile in Darwin, I've jumped out of a plane near Sydney, I've bungy-jumped towards a rainforest pool near Cairns and I've scrambled through flooded canyons in the Blue Mountains. None of them have a patch on something that should be basic child's play - climbing a tree.
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But the Dave Evans Bicentennial Tree is no humdrum back garden sun-blocker. It is surrounded by some of the tallest trees on earth - the centuries-old karris of south-west WA - and nonchalantly contrives to peek out above them. From top to bottom, it's 75 metres, and the only way up is a series of worryingly unprotected metal pegs.
They were hammered into the side of the tree to celebrate Australia's bicentenary in 1988, but this wasn't a new idea. The Gloucester and Diamond Trees, which form a triangle around Pemberton with Dave Evans' hideous-but-beautiful monster, have been used as fire lookout trees for decades. The entirely reasonable theory is that if you can get above the forest canopy, you can see whether there are any fires in the distance. The problem is that someone has to get up there.
In the age of helicopters, it's bravado-filled, vertigo threshold-testing tourists who go up rather than conscientious firefighters. And judging by the graffiti carved into the trunk at the first platform, an awful lot of people have been cajoled and chivvied into giving the test of nerve a go.
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The Dave Evans Bicentennial Tree is 65m high and is the highest treetop lookout in the world. Climbing this tree is not for the faint-hearted, however, those who do venture up the 182 pegs to the top will be rewarded with 360-degree views of the karri forest and glimpses of the Yeagarup Dunes and coast beyond. I did it ??????????.
A photo posted by Udo Gr&auml;dler (@backpackinglife) on Oct 28, 2015 at 3:45am PDT
That first platform is 25 metres up, and that's far enough to have the heart pounding, the mouth dry and limbs involuntarily shaking. It's not just about the height - it's about the overarching sensation of it not being at all safe.
There are no expert guides who have done it all before and will take care of the technical bits. There's no harness. There's nothing to clip on to. And there's nothing to catch you if you fall. The only nod to health and safety is some chicken wire forming a pitiful barricade along the end of the pegs. Otherwise, it's a spiralling ladder of easily-envisaged doom. Miss a step, and your foot is going to plunge downwards into thin air. Lose balance, and you're likely to barge through the chicken wire for a calamitous tumble.
It's this knowledge that one mistake can be lethal, and that it's entirely up to you to not make any mistakes, that makes the whole experience so fearsome. And that's probably why no-one's ever died doing it - the survival instinct has an incredible ability to trump natural clumsiness when sure-footed concentration is required.
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If the trip to the first platform is bad, the climb to the cabin at the top is gut-wrenchingly awful. The spiral goes nigh-on vertical, and the stakes rise with the height above ground. Once at the top, there's a temptation to just slump on the wooden platform, panting and weeping. 
If, however, there is any fight left in the legs by this stage, there are indeed some tremendous views. It's a horizon of treetops, showing just how relatively untouched by man this part of the world is. On a clear day, it's possible to see for 40-odd kilometres and the birds are flying beneath you.
This is a moment of respite to be treasured, because the one thing that truly makes this the scariest??? experience in Oz is that you have to relive it on the way down. And going down is far worse.
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There is something about big trees that just makes me want to climb them. This one was 65m tall and had a little ladder spiralling around it, the view from the top was amazing.... The drop between the ladder runs was daunting! #amazing_wa
A photo posted by Tom Rex Jessett | Australia (@trex.photography) on Feb 18, 2015 at 2:41pm PST
On the way up, it's possible to look partly at the horizon and partly at your feet - it's easy enough to get a feel for where the next peg is. On the way down, this isn't possible. There's only one way to look, and that's straight down where your feet are going.
Every step is accompanied by an internal monologue that screams: "Why did you do this, you idiot? What's wrong with you?" Shakes are now full-blown tremors. And there's no room amongst the maelstrom of blessed relief for the feeling of triumph to kick in once finally back on terra firma. That comes later, when the actual experience feels more like a distant bad dream - and you're far enough away to not have to go back up again.

More information
The Dave Evans Bicentennial Tree - 165 pegs of hell - is inside the Warren National Park near Pemberton. You'll need to buy a National Park Pass ($12 per vehicle) from the Pemberton Visitor Centre first. The tree is around 15 minutes' drive from the Visitor Centre. See pembertonvisitor.com.au.
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