We had left the brown bears below us in the Valley of the Geysers, where they ambled among purple orchids, emerald grasses and the second-largest gathering of geysers on Earth.

Now, our small group of trekkers ascended through meadows gilded with golden rhododendron. We drank from snowfield streams. Spectacular volcanoes loomed ahead, part of the great arc of volcanic and seismic activity known as the Pacific Ring of Fire. We were trekking the highlands of Russia's Kronotsky Zapovednik, diadem of the world's largest system of strictly protected nature reserves.

And I was one of the first foreigners permitted to go hiking here as a mere tourist.

Ever since Russian naturalists started the country's system of zapovedniki (strictly protected nature reserves) in 1916, these vast landscapes have been mostly inaccessible, except to scientists, rangers and students. But now, thanks to government initiatives begun in 2011, many of these reserves are opening to a limited number of travellers.

It is as if the US national park service (also, incidentally, founded in 1916) had forbidden hiking for most of a century – then decided a few travellers deserved a chance to explore the Grand Canyon or get close to Yellowstone’s Old Faithful.

When I heard, I came. Located on Russia’s 1,200km-long Kamchatka peninsula, which swings like a sabre away from Asia and toward the Americas, this nature reserve is closer to California than to Moscow. Although larger than Yellowstone, with more than 10,000sqkm of protected wilderness, Kronotsky for years has permitted only the most constrained of non-scholarly visits: after taking a helicopter flight costing 36,000 rubles, groups of tourists parade on boardwalks in small areas for under three hours. No roads go to Kronotsky.

Now, as our group of three reached the mid-point in a five-day hike, snow-crowned Kronotsky Volcano rose before us, lifting more than 3,500m from the nearby Pacific. At its base, 30 million landlocked salmon swirled in its lava-dammed lake, a buffet for hundreds of the world's best-fed (and, weighing up to 700kg, largest) brown bears. Kamchatka snow sheep roamed unmolested in the volcano’s heights – a privilege, since foreign hunters pay handsomely to shoot this long-horned subspecies in unprotected mountains 150km north of the sanctuary.

But nobody hunts here. The Russian word zapovednik comes from zapoved (commandment), as in "thou shalt not harm". The reserve's 33-year-old director, Tikhon Shpilenok, who came here after he worked as an anti-poaching ranger at another zapovednik, happily cited Russian law that permits only "educational tourism" in Russia's 102 zapovedniki.

Kronotsky's great educators are its long-time rangers, who escort trekkers in this almost-untracked land. My guardian ranger, Evgeny Vlasov, strode ahead of our small group, making himself visible in order to alert bears. For safety, Vlasov packed his shotgun shells with handmade rubber bullets – designed to stun but not harm a bear – that he carved from flexible gaskets, castoffs from a nuclear submarine base. Until 1991 and the easing of Cold War tension, the Kamchatka peninsula was so militarised that Russia prohibited all travel here by foreigners.

I had first met Vlasov 12km west of here, when a helicopter dropped me near his ranger cabin in Uzon Caldera, one of Kronotsky's botanical and geological wonderlands. From our first steps, he started teaching. The roots of the garnet-red Kamchatka lilies were savoured by his ancestors, Itelmen natives living here for millennia before Russians arrived in the late 1600s. And what looked like milk-white morning glories undulating in hot springs actually were swirls of heat-happy bacteria – among the many living "extremophiles" that draw scientists from around the world, probing for clues to early life's evolution.

Vlasov's polished-wood staff, which he extended often to help me leap across streams, was whittled from the stone birch trees that abound in the caldera, creating what looked like white-washed groves for hobbits. (When 1970s Soviet filmmakers created their version of an escapist journey to a Russian Shangri-La, Sannikov Land, they filmed here). As my legs began fading in the tenth hour of our first full day of backpacking – climbing out of one caldera, tracing a tufa-banked river and then descending into a gorge – the leaves that Vlasov shoved in my mouth were Itelmen favourites for fixing low testosterone. Or something. Specifics aside, the uplift got me to our next cabin ahead of darkness and bears, though Vlasov had to fire his shotgun skyward when he surprised a mother and two cubs on our trail near nightfall. All three vaulted upslope and away.

Now in the highlands with Kronotsky Volcano slipping out of sight, our hiking destination came into view: a ranger-patrol cabin that Vlasov built in 1985. Above it hulked a black-crested volcano called Savich. Born in an eruption near 700AD (making it roughly one-tenth the age of Kronotsky) and still fuming, Savich rides a blob of upwelling magma. Pressing up from about 8km beneath our feet, the blob drives superheated water through underground channels to create the Valley of the Geysers’ frothing fountains and bubbling mudpots.

From the ranger-patrol cabin we looked down into another of the magma blob's creations: a diabolic cousin to the green geyser valley, discovered only in 1975 when it received the name Death Valley. On windless days, its gases – laced with hydrogen sulphide and cyanide – settle near ground level and kill. Its victims have included foxes, bears and even yellow-beaked Steller's sea eagles on the rare occasion they stray from the fine fishing along the zapovednik's Pacific shoreline, less than 20km southeast of us.

Today, though, brought blue-sky breeziness with good odds for safety. Vlasov led us down slope, across sands of sulphur green and chrome yellow, into Kamchatka's vivid vale of death. We reached a lone set of human boot prints. I thought of Robinson Crusoe. "Slava", said Vlasov, speaking of a ranger who walked here from the geyser valley on routine patrol.

Beside a stream flowing over olive-green cobbles, Vlasov zipped to the top of a peaked boulder, raising his birch staff in mock-alpinist triumph. I bent low to photograph the olive stream – a mistake. My head reeled. Wooziness, rangers say, signals the onset of deadliness.

Vlasov, however, was already leading us out of our toxic gorge. "I survived Russia's Death Valley" stuck briefly as a bumper-sticker on my mind. Stretching out on a ridge to enjoy the breezes blowing through bellflowers and heathers, we gazed at the well-defended wilderness surrounding us.

Protecting Russia's wilds

For decades Russia's system of zapovedniki has provided the world's highest level of nature preservation (called category 1a by the International Union for the Conservation of Nature) to the largest land area within one nation. Nature protection for zapovedniki is stricter than for the world's national parks, some of which, like Yellowstone, have paved hundreds of kilometers of roads and permitted development of grand hotels within their boundaries. It also is stricter than for designated "wilderness areas" – like Alaska's Denali Wilderness, home to North America's highest peak and, at 8,600sqkm, almost as large as Kronotsky – which place few limits on hiking.

To Vlasov, Russia’s history of nature protection began long before 1916. As we dined on ranger-concocted potato soup and fish paste bruschetta, he reminded us that his Itelmen ancestors reputedly had begun preserving off-limits forests and no-hunting regions before the first Russians came here in the 1600s.

Today, the zapovednik system is a source of Russian pride. Just five days before my arrival in July 2012, Russian Prime Minister Dmitry Medvedev had helicoptered into the geyser valley and then shared some photos of his trip on Facebook. He got more than 3,800 "likes" for a misty snap of an iconic array of geysers, rising up a multi-toned valley wall known as "Stained Glass", tinted with reds, yellows, greens and blues by a mix of soils and plants. Medvedev's geyser junket followed a 2010 visit of then prime-minister and now president Vladimir Putin. Each leader, after visiting, announced support for expanding the zapovednik system by 11 new reserves from 2010 to 2020.

Prime Minister Medvedev did not stay overnight; most visitors cannot. Although Kronotsky is opening to a few hikers, its scientists have set limits on how many walkers may travel a route, in order to assure that soils remain undamaged and animals unharried.

Kronotsky's typical tourist, often dressed in Moscow-style street clothes, steps off a helicopter for about two hours of walking on boardwalks built by Vlasov and his ranger colleagues to protect both fragile plants and feet vulnerable to steam-searing on magma-heated earth. Even these visitors, however, are scarce: fewer than 4,000 short-stay heli-tourists visited Kronotsky in 2012, paltry in comparison to the nearly three million who visit Yellowstone's Old Faithful. Tourists allowed to overnight in Kronotsky, as I did, totalled 23.

Bear-chasing and geyser-gazing

For my last zapovednik evening, another ranger, Kolya Solovev, invited our half-a-dozen-strong geyser valley community to his cabin to dine on smoked salmon and watch a slideshow of polar bear and musk oxen, whose company Solovev enjoyed the year before when rangering at the Arctic zapovednik that protects all of Wrangel Island.

First though, Solovev had to deal with a young bear, about 2m tall with handsome claws, that the rangers had started calling "Freddy Krueger" in homage to the razor-clawed slasher from the horror film A Nightmare on Elm Street. Our Freddy had begun snuffling close behind groups of heli-tourists, perhaps smelling snack food. Rangers worried that Freddy might inadvertently hurt someone, reminding us all that even a slight increase in tourism can change wild animals' behaviour – and giving reason to limit tourism's reach in any zapovednik.

Freddy had ambled onto a helipad from which the day's last visitors had just flown, so Solovev loaded a shotgun with his preferred ammunition for bear safety: flare cartridges, designed only to scare. As Freddy veered toward a visitor centre, Solovev fired. Red flare-balls flew. Freddy scooted for the birches.

With peace restored to the valley, Solovev suggested a stroll before dinner. We followed the Geyser River toward the vaulting spouts and red-yellow soils of the great Stained Glass geysers. Earlier among heli-tourists I had jostled toward Stained Glass – a mixed pleasure, like glimpsing shards of the Rose Window of Chartres amid tour bus travellers.

Now with Solovev, his shotgun slung over his charcoal black t-shirt, we left the heli-tour boardwalk, wandered the river's gravel shore and sat on brick-red rocks at a pulsing spring called Malachite Grotto. Against a deepening blue sky, geysers puffed white.

One little puffer called Bastion, launching out of a 3m grey cone, spurted onto a grassy verge. Double Geyser, just up river, flipped crisp blips of water that slithered downhill on a brick-red dome whose colours recalled the ruddy top of the Duomo in Florence. Another dozen geysers and hot springs played around us. Up ahead was Averyev, a geyser I had been looking forward to see. It was named for a Russian volcanologist who battled in the 1960s to keep this valley preserved, after years when Soviet administrations in Moscow had tried to strip protection from many zapovedniki. Closer to us, Fountain Geyser blew, launching Stained Glass's largest tower of whitewater 10m into the air.

Beneath Stained Glass, we sat before the globe's greatest natural calliope – a steam-powered organ driven by the earth's heat – each blowhole piping its own pitch in our now-private sanctuary. As rangers kept telling me, geyser beauty gets greater after the last helicopter leaves and evening light turns slant.

Looking along the sights of his shotgun, Solovev took casual aim across the Geyser River, as if he had something to defend. And of course he did: one of the long-hidden glories of the globe, accessible now to small numbers of hikers – so long as they abide by a crucial zapoved, "thou shalt not harm".

Practicalities

Now that Russia's leaders are encouraging zapovedniki to bring a few overnight visitors for educational tourism, planning a hike in Kronotsky can be surprisingly easy.

All paperwork and logistics can be arranged by the Kamchatka-based tour operator Explore Kamchatka, run by an American and Russian couple, Martha Madsen and Yuri Barada. They can meet travellers right at Kamchatka's airport and offer a superb base at their bed and breakfast, which is surrounded by a small organic farm.

All hikers must travel with a ranger, be in good physical condition and be able to carry a substantial backpack. Hikers who do not speak Russian will gain from a bilingual guide like those who work with Explore Kamchatka.