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Alaska Division of Forestry via AP) MANDATORY CREDIT, Cameron Winfrey
In this Wednesday, May 5, 2016 photo provided by the Alaska Division of Forestry, a wildfire believed to have started from underground coal seams burns near Healy, Alaska. Pockets of ignited coal smolder and burn in the area all year and last year ignited nine wildfires. The Alaska Division of Forestry used a helicopter to drop water on the fire to keep it contained within an old burn scar.

ANCHORAGE, Alaska — Underground fires in coal country near the interior Alaska community of Healy have again touched off wildfires on the surface, putting forest officials on alert to keep them from spreading to populated areas.

Nine coal seam fires last year burned almost 800 acres, or about 1.25 square miles. Burning coal has lit four wildfires this year.

"It's sort of an annual rite of spring or early summer," said Tim Mowry, a spokesman for the Division of Forestry in Fairbanks. "When conditions are right — it's warm, it's dry, it's windy — they can sometimes come to the surface."

The largest wildfire has burned 640 acres, or about 1 square mile.

A 2010 research paper by University of Alaska Fairbanks and Division of Forestry researchers said spontaneous coal combustion is believed to be the primary natural cause of most coal seam fires. Lightning and forest fires cause others.

Coal has been mined near Healy since 1918. Usibelli Coal Mine has been in the area since 1943 and the underground fires are near its lease area 115 miles south of Fairbanks, mine spokeswoman Lorali Simon said by email.

"They maintain a consistent simmer, and if a big wind comes up, they can fan the flames and start a big forest fire," she said. In winter, Simon said, if there's a temperature inversion that acts like a cap to block normal air circulation, coal seam fires can smoke up valleys.

A coal seam fire was suspected in a 2009 wildfire that burned 254 square miles of boreal forest in 11 days, according to the UAF a research paper.

Wildfires started by burning coal seams come with an extra set of hazards for firefighters, Mowry said.

"You can't really do much with them," he said. "We don't want to send firefighters in there and have them fall into these coal seams. The ground could collapse around these things. It's not really safe to fight them on the ground."

They also emit harmful, noxious fumes.

"What we're left to do is monitor them from the air," Mowry said. "Or if we're going to take any suppression action on them, we're going to do that from the air by dropping, usually, water."

The good news is that areas near the underground fires have often burned before and there's not a concentration of highly flammable forest.

"We just try to keep them contained within these old burn scars so they don't get out and spread and threaten the community of Healy," Mowry said.