One late September day in 2012, Pat Sanders, lead interpretive ranger at Yukon-Charley Rivers National Preserve in Interior Alaska, heard an explosion in the distance.

"I wrote by email to a resident that lives downriver, asking if she was okay," Sanders recalled. "And she responded with, 'No, we're all dead -- why?'"

The neighbor might have been kidding around, but the cause of the explosion may turn out to be no laughing matter. A couple of weeks later, a fire was spotted about 25 miles northeast of the community of Eagle, a town of less than 100 accessible by road only during the summer months. After the first snow, an overflight of the fire revealed a smoldering patch of earth atop a peak, a growing caldera of unknown origin burning away, a jagged, black pit amid the pristine white snowfall.

That was Oct. 15, 2012, and it's been burning ever since.

"It looks like a warzone," Sanders said.

People in Eagle began to smell sulfur. The mysterious fire burning in the distance began to worry them. Officials started looking into it, dubbing it the Tatonduk slump and fire, or, alternately, the Windfall Mountain Fire. They determined the likeliest cause was an oil shale deposit under the mountain that had somehow ignited and was now burning steadily, growing as edges of the caldera collapsed into the black pit below.

Not volcanic

Marti Miller, the geology office chief with U.S. Geological Survey Alaska, said that though the activity may have a volcanic appearance, it doesn't appear to be volcanic in nature. And though coal fires can also burn for years or decades -- such as the one in Centralia, Penn., that long ago turned that community into a hazy ghost town -- previous geological mapping would indicate this isn't a coal-related fire.

"When we looked into it, we looked at the geologic map for that area to figure out what the rocks might actually be, to figure out what this event could be related to," Miller said. "From the geologic information, it looked like the area might be underlain by oil shale, not by coal. We also eliminated that it might be a hot-spring-type occurrence."

Miller qualified her statements by saying that USGS had not actually been able to visit the site of the fire yet. That's thanks in part to federal sequestration, which has seen the budgets of federal agencies slashed and hindered some operations throughout those agencies. She couldn't speculate on what started the fire, nor what might have caused the explosion heard in Eagle on Sept. 27, though she did say that underground coal fires could occasionally be sparked by lightning strikes, which is common in Interior Alaska.

Perhaps more concerning, though, is the sinister possibility that this underground fire could be emitting toxic gas -- namely, sulfur dioxide (SO2), an air pollutant that can also result from volcanic eruptions. In high doses, sulfur dioxide is toxic, and even in low concentrations, it can cause respiratory tract irritation.

Dangers of sulfur dioxide

At the moment, whatever gas may be in the air around Eagle doesn't seem to be causing any serious issues, said Dr. Ali Hamade, environmental public health program manager with the Alaska Department of Health and Social Services. He said his agency is advising people to treat the gas the way they might treat any other dangerous air pollution, including smoke from forest fires.

"The gas may be sulfur dioxide, and they also have wildfires up there, so depending on which way the wind is blowing," people could be affected, Hamade said. "We provided some house messaging for people to be guided by whenever there are bad air pollution days. For example sensitive or susceptible populations" -- the elderly, young children, asthmatics -- "should minimize activities, especially activities with prolonged exertion."

Hamade said that his department had checked with local health officials, who had not reported anything out of the ordinary with regards to the possible presence of gas.

Eagle's Mayor, Donald Woodruff, agreed with that assessment. As a small community, many Eagle residents wear numerous hats. In Woodruff's case, he's not only the town's mayor, he's also a first responder when the community's ambulance is called upon. He said he hadn't heard of any reports of particularly bad reactions to the gas, but still, he said, it's disconcerting.

He said that the smoke from the Windfall Mountain Fire is easily picked out from typical wildfire smoke, especially when the wind shifts from its normal direction and blows Windfall Mountain Fire smoke into the village on the southern bank of the mighty Yukon River.

"Lots of the smoke that's coming is coming from either Canada or the Tok area," Woodruff said. "But there are certain days where we get an upriver wind ... and you can see that band of nastyish, yellow-looking smoke coming upriver."

So far, Woodruff said, people have been able to use "common sense" and the same techniques they use to avoid wildfire smoke, like staying indoors as much as possible. Still, he admitted, "There are people that certainly are affected by the vapors, even 25 miles away."

But he's a little more concerned about the winter months.

Eagle is prone to serious temperature swings -- the community recently experienced 92-degree heat, and not for the first day this summer. In the winter, though, when the road to the community is closed and the residents huddle inside, the lows drop to 40 and 50 below zero regularly.

That can mean an inversion -- when the cold, often polluted air near the Earth's surface can't rise above the warm air over it, leaving it trapped and citizens breathing in those irritants. Inversions are the reason the Interior city of Fairbanks often has some of the dirtiest air in America during the winter. And Eagle is just as prone to such atmospheric headbutting. If some of that sulfur dioxide were to get into that lower layer of air, it could linger in the air Eagle's citizens are breathing in.

How bad will it get?

That cold air doesn't stop the fire from burning, though. Yukon-Charley ranger Sanders said she snapped a photo of the pit burning at 60 below zero last winter. Woodruff estimates the caldera has tripled in size since it was first photographed last autumn after the first snow.

It's burning hot, too. National Park Service geologist Linda Stromquist, who has able to take a trip to the fire site and put boots on the ground this summer, measured one fissure at 545 degrees fahrenheit, Sanders said. Stromquist herself was on leave and unavailable for comment.

Barbara Trost, air quality program manager with the Alaska Department of Environmental Conservation, confirmed Sanders' second-hand report that the surface temperature at the fire was quite high. She also said that Stromquist's trip to the Tatonduk slump confirmed the presence of sulfur dioxide on a sensor the team brought to the location.

The DEC is planning a trip up to Eagle in order to meet with members of the community and folks at Yukon-Charley Preserve, in order to determine the best location for an air monitor that can detect the presence of sulfur dioxide.

She said that they'll need somewhere with power, and someone in the community who can monitor the station, as well as a location where any sulfur dioxide is likely to be detected. "Depending on how the airflow is, when you get it to a point in the community where you can smell it, it's obviously causing at a minimum questions and concerns," Trost said.

Other sources of sulfur dioxide can include volcanic eruptions or emissions from power plants, but it's not a particularly common gas in Alaska. "We don't really have a network of SO2 monitors," Trost said. "We have one SO2 monitor working in Fairbanks at a multi-pollutant site in the community."

The DEC does have another piece of equipment that can also measure sulfur dioxide, and Trost said the agency was getting that ready for the trip to Eagle. "We have an older piece of equipment that's still functional," Trost said. "We're getting it set up here and getting it calibrated so it'll be ready to go once we figure out where we want it."

Despite the precautions, very little is known about the nature of the fire itself. How intense might it be? How long could it last? And if the problem persists, or if the fire grows, will the remote community of Eagle end up a ghost town itself?