There are plenty of trout swimming through the rivers that surround the isolated New Zealand village of Murchison, and anglers come from all over the world to try their luck. But the trout don’t run in the Blackwater River.

A seemingly typical clear and swift South Island river, the Blackwater has one unmistakable trait: the water carries a whiff of kerosene. It’s faint, but it was enough to give the Blackwater its name, and strong enough to keep the trout away. There’s oil buried in these hills, and in a deep fold of beech forest in the Blackwater Valley, the oil feeds a little-known natural phenomenon – if you know where to look.

Locals call it the Gas Blows, but Merve and Shirley Bigden – a small husband-and-wife tour operating team – have named it the Natural Flames Experience. Unknown to most travellers – and indeed to most people outside of Murchison, a two-street village 125km southwest Nelson – a bizarre cauldron of bright yellow smokeless flames burns eternally in the bush here, feeding off natural methane gas leaking continuously from the ground.

The bowl of flames has been burning, so the legend goes, since the 1920s, when a couple of hunters sat down for a smoke and one threw away his match, suddenly igniting the leaking gas right next to him. The Bigdens say the hunter actually sniffed the air and, in a moment of madness, tossed a light to see what would happen. Yet, another story says the hunters smelled gas, banged a pipe into the ground and then lit it. Regardless, the flames have rarely gone out since.

On a recent winter’s morning, I embarked on the four-hour Natural Flames Experience, with two other guests and local Shelley Neame as our guide. A bumpy 15km drive south led to a gentle, undulating 2.4km walk through the verdant bush. Neame is of old Murchison stock, tracing her family back to when the area was settled in the 1800s, and knows plenty of local history. She paused to tell stories about the trail and the area, pointing out an old rig that tried drilling for oil in the 1970s and describing the way the native plants have developed. She explained how the moa – now hunted to extinction but once as large as an ostrich – trampled through this bush, and she pointed out the bizarre horoeka tree, which evolved stiff, blade-like leathery leaves in its juvenile form to keep the moa away. Then, once it grew tall enough to be out of reach of the birds’ probing beaks, it evolved to burst into a tall tree topped with a cloud of green foliage. The trees still grow this way, nodding their lush heads at us as we walked along the track.

After about an hour, the bright yellow flames popped suddenly into view, cradled in a shallow bowl a couple of metres wide. Surrounded by a green swathe of damp native ferns and trees, the quietly spitting fire pit was totally unexpected. In fact, it felt eerie, as though bush spirits had abandoned the place just moments before. But on a cold and misty day, it was a welcome sight. I gazed in wonder, and then hurried closer to warm my chilly hands.

Neame dropped a camp pot over the flames, boiled water and added tea, which steeped until it was deep with smoky flavour. She then placed a skillet on the hot rocks nestled in the flames and cooked pancakes, offering us honey as a topping. It was local Tutaki beechdew honey, made from bees collecting the minute bubbles of honeydew that swell from the anal tubes of scale insects: tiny creatures that burrow into the trees and feast on the sap.

Despite that imagery, the honey was delicately sweet, the pancakes soft and steaming, and we chewed and gazed at the fire and wondered about the twist of geology deep in the earth that created it.

We sat on the rickety wooden benches on a small ledge above the flames for close to an hour, staring down at the fire nestled in the misty cool bush threaded with frost. It was astonishing to think it had burned almost continuously for nearly 100 years (though sometimes after a heavy rain, the gas needs to be reignited).

There are at least nine other spots around the world where eternal flames burn from natural gas. The two best known are major tourist attractions, sparking centuries-old tales of mythology and inspiring cultural and religious fervour. Yanartaş, on Turkey’s Mount Olympos, has dozens of small fires that burn continuously from gaps in the rock, bright enough to once guide sailors at night. They’ve been there for at least 2,500 years, and the area is said to perhaps be the origin of the myth of the fire-breathing Chimera in Homer’s Iliad. Below the fires are the ruins of the temple of Hephaistos, the Greek god of fire, metalworking, forges and blacksmiths.

Another eternal flame burns at India’s Jwalamukhi Temple in the Himachal Pradesh town of Jwalamukhi. It is worshipped as a deity and attracts thousands of pilgrims every year, who bring gifts of candy, milk and fruit.

New Zealand, a geologically and culturally young country, has nothing like that; its equivalent to the classical ruins of antiquity is its ancient bush, birds, waters and landscape. But this natural wonder is one-of-a-kind: nowhere else in the world do the flames burn undisturbed in the middle of thick, uninhabited bush. To make them even more special, the natural shifts of oil and gas under the earth, as well as the imminent prospect of drilling later this year, mean they could be gone tomorrow.

For now, though, the eternal flames and the small, husband-and-wife tour remain. It’s a chance to walk through lush beech and soft moss until you happen suddenly upon them, eventually leaving the fire still crackling and wondering at the power of unseen forces deep in the earth.