Mysterious underground fire has been burning near Yosemite for 5 years

U.S. Forest Service crews are conducting controlled burns to reduce fuels in the Rosasco Meadow area of the Stanislaus National Forest west of Yosemite, where a strange underground fire is burning. The fire has been smoldering for at least five years, and foresters aren't sure what caused it or how it's being fueled. less U.S. Forest Service crews are conducting controlled burns to reduce fuels in the Rosasco Meadow area of the Stanislaus National Forest west of Yosemite, where a strange underground fire is burning. The fire has ... more Photo: Carol Ewell/US Forest Service Photo: Carol Ewell/US Forest Service Image 1 of / 4 Caption Close Mysterious underground fire has been burning near Yosemite for 5 years 1 / 4 Back to Gallery

For at least five years, a fire has been burning underground in the Stanislaus Forest west of Yosemite, and foresters say they have no idea what started it or exactly how big it is.

The blaze is smoldering near Rosasco Meadow, roughly seven miles west of Yosemite National Park and 12 miles southwest of the town of Twain Harte. It lies under terrain scorched by the massive Rim Fire in 2013.

Groveland District Ranger Jim Junette, who oversees the Stanislaus Forest's southern area, told the Modesto Bee last week that that rangers are baffled by fire, which has consumed tree roots in a circular area with a 400-foot diameter. Junette said they aren't sure how deep the fire goes or what is fueling it because it's too dangerous to get a close look.

In 2015, the Union Democrat reported that the "Rosasco Fire," described by the Forest Service as a "holdover heat signature from the Rim Fire," was burning over 3.4 acres. The article did not mention it being underground.

Junette said wildfires sometimes flare up after a year, but never this long. He ruled out lightning as a possible ignition cause after checking weather records.

An old logging railroad grade winds by to the east of Rosasco Meadow. Junette told the Bee that it's possible that the remains of creosote-treated ties could be a fuel source for the fire.

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Subterranean fires have been known to burn for decades, but most are in coal-mining areas. For example, the Centralia mine fire has been smoldering for more than 50 years under the now abandoned town of Centralia in Pennsylvania's anthracite region.

Since the Rosasco Meadow fire has flared up at least twice since 2014, igniting dead vegetation and tree falls, U.S. Forest Service crews are performing controlled burns in the area to reduce woody fuels and lessen the chance of an out-of-control fire. The burns are reportedly expected to continue through Thursday.

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Mike Moffitt is an SFGATE Digital Reporter. Email: moffitt@sfgate.com. Twitter: @Mike_at_SFGate.