Ventura County firefighters are monitoring a patch of land north of Fillmore where the ground temperature climbed to 812 degrees on Friday for unknown reasons.

Possible theories include that natural hydrocarbons such as oil or gas are burning deep in the earth. But nobody knows for sure what might have ignited the materials.

“We are a little perplexed at this point, to tell you the truth,” said David Panaro, a geologist with the Ventura County Watershed Protection, who was one of a few scientists called in to help solve the mystery. “This is not your usual geological detective story.”

The area has recorded high heat at least five times since 1987, said Allen King, a retired geologist with the U.S. Forest Service, who also surveyed the scene where small streams of smoke wafted from cracks in the ground. The area is on land owned by the U.S. Bureau of Land Management and leased by an oil company.

On Friday, the Ventura County Fire Department cleared the brush near the fumes and cut a fire line around the area to ensure a fire didn’t start from the heat.

King said it’s possible the blaze was started by another fire on the ground and it ignited a second fire underground. It could be as deep as 100 feet, he said.

Such fires where there are high concentrations of hydrocarbons are not uncommon around the world, he said.