VIDEO: On a ridge overlooking the foothills of the Sierra Nevada Mountains, it would have been possible to see the first flickering flames of the Oak Fire and then how it engulfed its surroundings.
It took just 24 hours to mushroom to 10,000 acres and become California's biggest wildfire this year.
"That's crazy fast," said Joe Amador, one of thousands of firefighters from across the state now deployed to fight the blaze.
"In the initial stages it was wind-driven and terrain-driven," he told CNN, pointing to rolling hills, peaks and valleys that fire crews call "chimneys" that can help to spread embers and fire rapidly.
Winds were light on Thursday, allowing an inversion layer to form, which kept flames shorter and held down the thick smoke blanketing the hills outside Mariposa, close to Yosemite National Park -- a blessing for the teams trying to contain the fire and protect buildings. "We're in extreme conditions, but things can always get worse," Amador said ruefully.