It was a fire that crossed mountain ranges and valleys, that spanned multiple counties and shocked Californians by its sheer scale — by far the biggest wildfire in modern state history.

And yet a newly disclosed investigation suggests it was likely started by a single man and a single spark.

In a report released in recent days, forensic investigators found that a rancher started the fire when hammering a metal stake in his backyard to snuff out a wasp nest.

Sparks flew, igniting dry grass stalks and spreading fire quickly across the desiccated landscape.

The rancher’s name was not disclosed, but a review of records led to the home of Glenn Kile, a former heavy equipment operator in his mid-50s, who had no inkling of the devastation he would unleash on a Friday morning last July while tinkering in his backyard. Seeing the fire, he said, came as a shock.

California in flames: wildfires rage across the US state in 2018 Show all 45 1 /45 California in flames: wildfires rage across the US state in 2018 California in flames: wildfires rage across the US state in 2018 Firefighters try to limit the spread of the Carr Fire by backburning, a process by which areas in the path of a fire are burned up in advance to rob the fire of its fuel Getty California in flames: wildfires rage across the US state in 2018 A street is devastated by the Carr Fire in the town of Redding AP California in flames: wildfires rage across the US state in 2018 A truck burned by the Delta Fire is abandoned along interstate 5 AFP/Getty California in flames: wildfires rage across the US state in 2018 This image from the National Oceanic and Atmospheric Administration highlights the major fire AP California in flames: wildfires rage across the US state in 2018 The Ranch Fire spots out ahead of the main fire in Spring Valley, burning two homes AP California in flames: wildfires rage across the US state in 2018 A picture taken by German astronaut and geophysicist Alexander Gerst, showing wildfires in the state of California as seen from the International Space Station AFP/Getty/ESA/Alexander Gerst California in flames: wildfires rage across the US state in 2018 Evacuees from Lucerne, from left, Ken Bennett with Ember Reynolds, 8, and Lisa Reynolds watch the sunset as smoke from the Ranch Fire rises into the sky at Austin Park Beach in California's Clearlake with Mount Konocti in the background The Press Democrat via AP California in flames: wildfires rage across the US state in 2018 The memorial for Redding firefighter Jeremy Stoke AP California in flames: wildfires rage across the US state in 2018 Aerial view of Trabuco Canyon as a tanker aircraft dumps load onto Holy Fire, Near Santiago Peak, California, U.S., August 6, 2018 in this still image taken from a video obtained from social media. 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MANDATORY CREDIT SOCIAL MEDIA REUTERS California in flames: wildfires rage across the US state in 2018 Holy Jim volunteer firefighter Luke Senger stands next to a home destroyed the Holy Fire in Trabuco Canyon AP California in flames: wildfires rage across the US state in 2018 Firefighters try to stop the spread of a massive fire in Lake County AP California in flames: wildfires rage across the US state in 2018 A woman walks out of the water after taking a dip as smoke rises in the distance from the Holy Fire in Cleveland National Forest Getty California in flames: wildfires rage across the US state in 2018 A pyrocumulus cloud (or cloud of fire) explodes in the Carr fire near the town of Redding Getty California in flames: wildfires rage across the US state in 2018 The Delta Fire burns in the Shasta-Trinity National Forest AP California in flames: wildfires rage across the US state in 2018 Trees set alight near Whiskeytown Getty California in flames: wildfires rage across the US state in 2018 An firefighting aircraft drops fire retardant to slow the spread of the River Fire near Lakeport Getty California in flames: wildfires rage across the US state in 2018 A firefighter lights backfires in an attempt to limit the spread of the Ranch Fire Reuters California in flames: wildfires rage across the US state in 2018 Firefighters light backfires to defend houses in the town Upper Lake from the Ranch Fire Getty California in flames: wildfires rage across the US state in 2018 Flames consume a home in Lakeport AP California in flames: wildfires rage across the US state in 2018 The Ranch Fire spread towards the town of Upper Lake AFP/Getty Images California in flames: wildfires rage across the US state in 2018 A firefighter lights a backfire in the city of Redding Getty California in flames: wildfires rage across the US state in 2018 The Carr Fire burns near Whiskeytown AP California in flames: wildfires rage across the US state in 2018 Firefighters try to limit the spread of the Carr Fire by backburning Getty California in flames: wildfires rage across the US state in 2018 Flames flare in the Cajon Pass near San Bernardino AP California in flames: wildfires rage across the US state in 2018 An firefighting aircraft drops fire retardant to slow the spread of the River Fire near Lakeport Reuters California in flames: wildfires rage across the US state in 2018 A schoolhouse burns in the Carr Fire in Shasta County AP California in flames: wildfires rage across the US state in 2018 The Carr Fire burns along Highway 299 by the city of Redding AP California in flames: wildfires rage across the US state in 2018 A car is seen amongst the devastation left by the Cranston Fire EPA California in flames: wildfires rage across the US state in 2018 A firefighter waters down a backfire on Cloverdale Road in Igo AP California in flames: wildfires rage across the US state in 2018 Firefighters convene while battling the Carr Fire in Shasta County AP California in flames: wildfires rage across the US state in 2018 Firefighters assess the damage in San Bernardino County AP California in flames: wildfires rage across the US state in 2018 The Carr Fire burns along Highway 299 by the city of Redding AP California in flames: wildfires rage across the US state in 2018 The Carr Fire burns along Highway 299 in Shasta County AP California in flames: wildfires rage across the US state in 2018 The land around Mountain Centre has been left devastated by the Cranston Fire EPA California in flames: wildfires rage across the US state in 2018 Firefighters battle the Carr Fire in the town of Redding AP California in flames: wildfires rage across the US state in 2018 A home burns in Redding Getty California in flames: wildfires rage across the US state in 2018 The Cranston Fire burns south of Idyllwild AP California in flames: wildfires rage across the US state in 2018 The Carr Fire burns through the town of Shasta AP California in flames: wildfires rage across the US state in 2018 A home burns in Redding Getty California in flames: wildfires rage across the US state in 2018 A collection of antique cars has been destroyed by the fire in the town of Redding Getty California in flames: wildfires rage across the US state in 2018 A home on Sunflower Road in Redding burns in the Carr Fire Getty California in flames: wildfires rage across the US state in 2018 A helicopter aids utility workers as they work to repair burned power lines EPA California in flames: wildfires rage across the US state in 2018 A home burns in Redding Getty California in flames: wildfires rage across the US state in 2018 The Carr Fire burns along Highway 299 by the city of Redding AP California in flames: wildfires rage across the US state in 2018 Sunflower Road in Redding burns in the Carr Fire Getty

“I smelled smoke, I turned around and there it was,” Mr Kile said in an interview on the porch of his barn-red, two-story home this week.

Mr Kile, who hasn’t talked publicly about the fire before, referred to it as if it had been an apparition. “There was nothing I could do,” he said.

During a morning of chores on his ranch three hours north of San Francisco, he had spotted an underground wasp nest.

He grabbed a metal stake and pounded it into the hole to try to seal it off, according to an investigation by California’s fire agency. He told investigators he was allergic to stings and wanted to plug the hole.

He was judged responsible for the fire, but not negligent. Under the heading of violations, the report says “Not Applicable”.

The ignition of the so-called Ranch Fire is a testament to the extreme fragility and volatility of the American West, fire experts say. California in particular is such a tinderbox that something as seemingly innocuous as hammering a stake into the ground can unleash an uncontrollable inferno.

Thousands of firefighters worked for more than a month to extinguish the Ranch Fire, which destroyed more than 150 homes and cost tens of millions of dollars to suppress.

The fire burned 410,203 acres of California wild lands, an area half the size of Rhode Island, and killed a firefighter who was struck by a falling tree.

While it was one of the largest fires, it was far from the deadliest. The Paradise Fire in the fall of 2018 killed more than 80 people.

“It really drives home how volatile conditions are,” said Ken Pimlott, a career firefighter who oversaw the Ranch Fire as chief of the California Department of Forestry and Fire Protection, better known as Cal Fire.

Mr Pimlott, who retired in December, said that walking through a California meadow in summer was akin to wading through a pool of gasoline.

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“We have to have a healthy understanding that this grass that I’m standing in won’t take much — won’t take anything to start,” he said.

With temperatures reaching into the triple digits in some parts of California this week, anxiety about fires is again rising.

After a wet winter that drove the growth of vegetation across the state, many Californians live amid wild lands that scientists say are becoming increasingly hot and dry.

Governor Gavin Newsom declared a state-wide emergency in March, bypassing environmental rules to speed up the clearing of vegetation.

But the fires of the past two years — the wine country fires in Napa and Sonoma counties and the inferno that all but razed the town of Paradise — have shown that some wind-driven fires are so intense that firefighters are helpless to stop them.

The 2017 Tubbs Fire that ravaged parts of Santa Rosa jumped an eight-lane-wide freeway.

Power lines and carelessly discarded cigarettes have been among the most notorious culprits of wildfires in California.

But Mr Pimlott said many fires started without any negligence: a grandson who helped his grandfather with a gasoline-powered weed eater; a driver who towed a boat on a highway that lost a wheel and scraped the ground, sending sparks flying; a worker welding a gate at a church retreat.

Fireworks, campfires, cars overheating and children playing with fire are also common causes of fires, as are electricity transmission lines, officials say.

In May, Cal Fire found that Pacific Gas & Electric (PG&E), the giant Northern California utility, was responsible for starting the Camp Fire, one in a string of fires that authorities have blamed on the company. PG&E filed for bankruptcy in January, citing wildfire liabilities.

In the case of the Ranch Fire, Mr Kile, who said he was forced into early retirement by a back injury, was planning to erect plastic sheeting to shade water storage tanks from the sun when he came across the underground wasp nest.

He seemed more bewildered than remorseful about starting such a massive fire. “Mother Nature,” he said, “you have no control”.

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He said at first he tried to stop the fire by throwing a nearby trampoline and an old carpet on it; he shovelled dirt on the flames and then tried to douse them with water from a hose that melted and would not unkink.

Then, according to the Cal Fire report, “he unhooked his trailer and tried to put the fire out by ‘kicking up dirt’ ahead of it with his four-wheeler.” Moments later, “He lost control of his four-wheeler which rolled downhill and lodged between the water tanks and the cut bank.”

Finally, when nothing worked, he ran down the hill and dialled 911.

Firefighters responded with trucks and airdrops of flame retardant. According to the report, Mr Kile turned on the water pump near his residence to supply the water tanks to help the firefighters.

At the same time, he was suffering from what appeared to be smoke inhalation and heat exhaustion. He refused an ambulance and medical treatment.

Ron Milliken, a neighbour across Highway 20, which runs below Kile’s house, took a picture of one of the planes dropping retardant chemicals at 12:46pm, less than an hour after Mr Kile dialled 911.

The Cal Fire report detailed the steps investigators took to analyse the cause. Using magnets and tweezers, they found tiny metal shards near the wasp nest.

Any one of them, or all, could have ignited the dry grass. The metal stake was misshapen where it had been struck by the hammer.

Another fire that ignited on the same day to the southwest was called the River Fire. Because of their proximity and timing, the two fires were together called the Mendocino Complex Fire. Collectively they burned around 460,000 acres.

Above the house, a steep hill climbs to the rim of the valley, covered in tall grasses and oaks. Today, the scars from the Ranch Fire are still visible along Highway 20. Shrubs and small trees appear as blackened silhouettes.

But the taller oak trees survived and the grass grew vigorously during the spring rains. On Mr Kile’s 160-acre ranch, the grass is nearly waist high — and dried up.

Mr Kile says he is concerned that fire could return, adding that he is thinking of heading to the more humid coast during the driest and hottest days.

“Every time I turn the radio on there’s another fire started here or there,” he said. “I need to get away.”