The Fillmore underground fire is a patch of ground on private property just outside the Los Padres National Forest, in Ventura County, California. Burning at around 800 degrees in spots, the subsurface burning is likely fueled by hydrocarbons – oil and associated gases such as methane. The unstable ground allows oxygen to enter to sustain the fire. First noticed in the spring of 2008, a wildfire in 2007 may have ignited it. Or, it may be the result of exposed sulfide minerals in the underlying rock. When air reaches the buried shale that contains minerals, such as pyrite and marcasite, the minerals oxide and produce heat. Nearby organic material can catch on fire and the petroleum products in the rock and soil feed it further. The continuous heat creates surface fires during dry times of the year and firefighters must be vigilant in this area. The hot spot, called a “thermal anomaly” is several acres and growing. Other underground fires have also started in this area. When the burning is not just from dry vegetation but from hot gases emitting from the ground, it becomes more difficult to extinguish and a chronic problem.

Several people compare this underground fire to that of Centralia, Pennsylvania. However, the Centralia disaster was the result of careless burning against a coal seam that caught on fire – a situation that is difficult to replicate in modern times. Surface fires no longer occur in Centralia related to underground burning. Fillmore is likely entirely natural and a form of spontaneous combustion that is difficult to fully extinguish and may periodically reoccur due to the dry climate and surface instability.

Wisps of smoke and even a burning smell can sometimes be detected emanating from the Fillmore thermal anomaly.

Sources
The earth is on fire above Fillmore, and the underground burn has gotten bigger
Underground fire continues to burn north of Fillmore
Spontaneous Combustion, Underground Fires and Why Scientists Can’t Fully Figure Out This Wildfire Concern

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