Get yourself a backcountry pass, pack your bear spray and head into the true wilderness of Yellowstone NP, Wyoming.
Two days and as many river valleys behind us, I wriggle out of my pack and pause to take in our surroundings. With Susie and Dan a few minutes behind, the silence is overwhelming. Tracking upstream along the Yellowstone River, we had entered an area ironically referred to as the Thorofare.
Ironic, because this is in fact the most remote area in the Lower 48 United States - an expansive open river valley eventually emptying into Lake Yellowstone - otherwise bounded by mountains, not especially high, but imposing nonetheless.
Another 20 minutes on and we spot some odd shapes in the autumnal hues of the alluvial meadow. Two bison. Our first. That day we had met the one and only person we were to see for the first five days of a six-day walk - a cheery ranger who quipped that we, too, were the first folks he'd seen in a week...
We were in the States for five weeks and had planned some relaxation, interspersed with an eclectic mix of mini-adventures involving various combinations of climbing, scuba, cappuccinos, wildlife and volcanoes.
Susie, my partner in crime, had acquiesced enthusiastically to most of these schemes, but being a lawyer-cum-hiker, insisted on a mandatory clause of a bushwalk of not less than one week in duration.
Our friend Dan was a late but welcome addition - eager to capitalise on having leave before immersing herself in orthopedic surgery training.
Yellowstone National Park: history
Lying in the heart of the Rockies - mostly in the north-west corner of Wyoming - Yellowstone National Park and World Heritage site spans 8987sq.km and protects a fascinating complement of wildlife, volcanic features, lakes, canyons and rivers.
French-Canadian fur trappers were the first white people to reach the area, 11,000 years ago. The famed Lewis and Clarke wanderings of 1806 passed just to the north, and the first dedicated expedition in 1869 by Folston, Cook and Peterson named what is now the world's most celebrated geyser - Old Faithful.
Later, in 1872, Yellowstone was "dedicated and set apart as a public park or pleasuring ground for the benefit and enjoyment of the people". And so was proclaimed the world's first national park.
Three million visitors a year to Yellowstone is not unusual. Empty, seemingly unending wilderness is probably not something most Americans would associate with Yellowstone. But venture backcountry and that's exactly what you'll find.
The dichotomy between it and the major road-accessible attractions is mind blowing. Yellowstone is almost completely bounded by designated national forest wilderness areas, wildlife refuges and another national park - Grand Teton.
All these protect a large part of the Greater Yellowstone Ecosystem, providing an invaluable buffer for the parks' wildlife.
Wildlife of Yellowstone
Having rather foolishly slept in on our second morning, we had to cover some serious ground on day three. That afternoon, the frigid Yellowstone River proved mercifully shallow, but we realised that we'd end up walking at night.
Given that the tracks are pretty well marked you'd think a modicum of ambling under the stars might actually be quite romantic. But, in addition to being somewhat tired, one is warned against walking at night. We could see why.
For almost the entire first five days of our walk, we saw no human footprints. There were plenty of others: deer, elk, coyote, wolf and, most common, bears. Claw marks on trees, prints, and scats, often unnervingly fresh, were a constant reminder that we were in their territory.
Grizzlies - once found over most of the west side of the USA down into Mexico - in the contiguous USA are now restricted to a few areas in the north. They've fared better in parts of Canada and Alaska. Thankfully, though, Yellowstone and its surrounds provide sanctuary.
Suffice to say we made so much noise that evening that our only run-in was with a distant mule deer. There are few animals that embody the word 'wild' more than the wolf.
Deliberately targeted, the last grey wolf disappeared from Yellowstone in the mid-1920s. In one of the most publicised wildlife restoration actions to date, wolves were reintroduced from Canada in 1995 and 1996.
At the end of 2009, there was only an estimated 98 wolves in the Greater Yellowstone Ecosystem. Like all predators, they need space and sufficient prey. The reintroduction has had a complex but significant ecological impact though.
Numbers of elk (also known as red deer, the wolf's main prey) in northern Yellowstone have declined markedly from a high of around 19,000 in 1994, to 9500 in 2005, down to 6000 at the last count. The elk population fluctuates greatly anyway, due to hunting and harsh winters, but there's no doubting that this predator is doing its thing.
Just over halfway through our journey, we traversed Two Ocean Plateau - so called because it drains into both the Pacific and Atlantic. We then connected with the southern end of Lake Yellowstone, where I was sure we'd finally meet up with fellow backcountry hikers.
Slightly ahead of Susie and Dan, I strolled out of some dense forest into a serene clearing, gently sloping down to the placid waters of the lake. The soft, dark soil was pockmarked with a miscellany of animal prints and a lone bald eagle stared down intensely from a dead lodgepole pine. It felt like no-one had been here in quite some time.
Of the three of us, Dan (no softie, I might add) was the most nervous about the wildlife. Later that evening as we made the finishing touches to our campsite and gathered firewood in anticipation of the inevitable plummet in temperature and roasting of marshmallows, the tranquility was abruptly shattered by a piercing, drawn out half-scream, half-roar.
Grinning mischievously, I looked over at Dan who'd stopped dead in her tracks, her mind, I found out later, racing with images of some hapless Bambi being ripped limb-from-limb by a grizzly, cougar or other unseen rapacious predator.
The reality was somewhat less dramatic, but no less interesting. Male elk at this time of year compete with one-another for mating rights, strutting their stuff and vocalising to show who's boss.
That night the gentle brushing of wind through the trees was punctuated with the clacking of antlers as their testosterone-filled bellows escalated into physical confrontation.
Fire and ice
Throughout most of Yellowstone's long history, fire was viewed as a singularly destructive force to be battled and hopefully vanquished. As time went on, opinion shifted as ecologists increasingly recognised wildfire as a natural process, to which many plants and animals are well-adapted.
This was part of a broader trend valuing the 'wildness' of Yellowstone - hot springs were no longer diverted for bathing, roadside feeding of animals was banned and open dumps were closed as bears had become reliant on them.
By 1972, the policy, which has since been nuanced but largely remains today, was for naturally ignited fires to be allowed to run their course except where threatening property. However, by the late 1980s about one-third of Yellowstone's forests were more than 250 years old and bearing high fuel loads - setting the scene for a major conflagration. A run of dry weather was all that was needed. Disaster struck in the summer of 1988.
After 249 fires in the Greater Yellowstone Ecosystem, two deaths, 25,000 firefighters called in to battle the flames, and a $120 million damage bill later, 36 per cent of the park had burned. Crown fire killed a huge number of the trees outright.
But a relatively tiny number of animals succumbed directly, and, years later, populations of most of the well-known species appeared unaffected. In fact, severe winters have much more pronounced effects on wildlife. But, be warned. Famously cold winters, coupled with a short growing season, mean that the most immediately noticeable feature for many areas in the park is dead trees.
Millions of them. This effect renders much of the forest scenery rather austere, bordering on post-apocalyptic in places, and this could be quite confronting if you're not expecting it. I saw a unique beauty, though, in the mute tones and geometry of the dead standing and fallen timber. Pine regrowth is vigorous in places, and totally non-existent in others. Life goes on. Even under the surface.
Yellowstone is a volcano, and 640,000 years ago, it really spat the dummy. That eruption spewed out pulverised rock and lava, with an angry, broiling column of ash punching 30km into the atmosphere. Eruptions on this scale empty the magma chamber beneath to such an extent that the overlying rock collapses into the cavity producing an enormous depression, called a caldera.
For more than 18 million years, successive eruptions of the Yellowstone hot spot gouged out the massive Snake River Plain which stretches right across southern Idaho. The current caldera spans an area 45x75km and, far from dead, the heat from the magma chamber below Yellowstone powers its many wondrous thermal attractions, which eventually ensured the park's protected status.
It produces thousands of earthquakes a year and constant buckling, bulging and subsidence of the caldera floor. There are more geysers in Yellowstone than everywhere else on Earth put together.
That night and our last campsite felt more like being back in civilisation as we remarked on the comparative luxury of a new bear box (not just a horizontal pole between two trees for hanging packs) and a proper drop toilet - albeit completely open to the elements. We started seeing other hikers on the last day, but also enjoyed our initiation into Yellowstone's thousands of thermal delights - boiling mud pools, fumaroles, hot springs and geysers.
Yellowstone's lofty neighbour
Whereas Yellowstone NP's mountains are undulating and a little stark, the Tetons positively burst skyward, looming authoritatively from the surrounding plains and lakes. With only one day in Grand Teton NP, our intention was to relax. However, I had a growing itch for one last adventure which I just had to scratch.
As Susie and Dan set off up the valley, I gathered my helmet and bear spray and started climbing like a man possessed. An unexpected, unmarked track saw me quickly reach the tarn I had initially set my sights on, so I just kept going. Three lung-bursting hours and some seriously steep gullies later, I could climb no further.
From my vantage point, I got to thinking how easy it would be to traverse into Yellowstone proper from here too... another trip maybe. I took a drink and, before starting the long descent, bid a private farewell to this extraordinary corner of the planet.