Under a patch of earth on a cliff above Little Sespe Canyon near Fillmore, temperatures have topped 800 degrees.
Deep beneath the surface, a fire continues to burn and smolder years after authorities discovered the underground blaze. In August 2008, gusts of smoke broke through the ground, prompting firefighters to clear brush from the hot spot and call in experts for answers.
Geologists and other scientists investigated, trying find the cause of what officials first called "a thermal anomaly," said Bill Nash, a spokesman with the Ventura County Fire Department.
"We've met with a number of people on it, really trying to figure out what it is," he said.
They prodded and tested the ground and the smoke and decided the fire likely started in an oil seam. What exactly ignited the blaze and how long it will last are less clear.
"It's been there for a long time, and it's just something we're going to have to continue to monitor," Nash said.
It's not the only one under Ventura County. In 2004, a patch of land northwest of Ojai grew hot enough to start a brush fire, burning 3 acres in the Los Padres National Forest. The spot has since been cooling off.
"It was hot enough to actually ignite some of the local vegetation there," said Scott Minor, a researcher with the U.S. Geological Survey in Denver. "When the firefighters came in to put it out, they realized it wasn't just wood burning. It was actually this hot gas coming out of the ground, which could not really be put out."
But other than the high temperatures underground, experts say the two fires have little in common.
Minor studied the Los Padres occurrence with other scientists, publishing a paper in late 2008 on their theory of what led to the blaze.
Their best theory states that unlike the hot spot near Fillmore, the Los Padres fire was related to an abundance of naturally occurring iron sulfide minerals. A landslide likely pushed in oxygen that helped ignite the blaze.
Such hot spots are "somewhat unusual," Minor said. "But they may be more common than we realized, at least in that part of coastal California."
Underground fires like the one in Fillmore are less mysterious, said Allen King, a retired geologist with the Forest Service who has studied both sites.
A colleague of King's came in 2008 and collected and analyzed gas emissions, which appeared to be petroleum-based, King said.
A 2007 wildfire may have ignited those gases, causing the blaze. What sustains the fire is unclear, but oxygen may be seeping in through cracks created by a previous landslide, King said.
Several miles past the end of Goodenough Road, outside of the city, the hot spot is on private property. Officials posted signs asking people to call 911 if they saw flames but not to worry about smoke.
The ground stays hot in the area, but temperatures vary depending on what is going on under the surface, Nash said.
"Our wild-land division checks on it regularly to make sure the brush is cleared away," he said.
At last check, wisps of smoke continued to break through the scorched dirt, giving a glimpse of the burning below, officials said. Authorities don't consider the hot spot a safety hazard but keep an eye on it to make sure that doesn't change.
"It's not a threat," Nash said. "But it's something we keep a watch on."
Fillmore hot spot (approximate location)
View Fillmore hot spot (approximate location) in a larger map