Not far from Hazard, Kentucky, in the shadow of Lost Mountain, a woman named Ruth Mullins saw smoke rising off the slope. “I knew it wasn’t no woods on fire, because of the smell”—the rotten-egg stench of sulfur—she said. Her suspicions were soon confirmed: Lost Mountain’s coal mine, abandoned for 40 years, was burning.

Kentucky names coal fires for the people who first report them, so the fire, which has continued to smolder and occasionally flame since it was identified in 2007, is known officially as the Ruth Mullins fire. “We’ve never met the woman and we don’t know where she lives, but her name now appears in scientific publications that are read all over the world,” says Jennifer O’Keefe, a geologist at Kentucky’s Morehead State University. “She’s got her little bit of immortality.”

O’Keefe is part of a team that has been visiting the Ruth Mullins fire over the past three years, studying its behavior and quantifying the gases that plume from nine known openings in the ground. Last January she and a colleague, University of Kentucky geologist James Hower, brought some students to the coal fire for new measurements. They parked off Highway 80, a road that cuts a swath along the side of Lost Mountain, and unloaded gear in a stingingly cold wind as speeding trucks whipped ice along the asphalt. Trudging up the snow-covered mountain, the scientists shivered along the flat shelf of land circling its midsection, the remains of contour mining in the 1950s. While smoke from the burning mine had been hard to spot from the road, here it billowed from small vents where portals to the mine had collapsed.

Approaching the site, all except Hower (who stayed farther back) donned pink respirators. A student equipped with a GPS device tried to detect the outline of the underground fire by looking for areas where the snow was thinner or melted away entirely. Two other students and O’Keefe settled at a vent, measuring the temperature at the opening and the velocity of the gases (including carbon monoxide, carbon dioxide, hydrogen sulfide, methane, and oxygen) that were flowing out.

“Jen, do we have any tar or minerals up there?” Hower called to O’Keefe, who shook her head. He made his way care­fully over some fallen trees, possibly killed by the coal fire cooking their roots, to another vent and climbed closer, sliding a little in the snowmelt and mud made warm by the mine’s hot breath. Here there was plenty of tar and minerals: Black goo two-toned the leaves on the ground, and minerals that had precipitated out of the gases encrusted the tree roots dangling over the vents. To identify the potentially dozens of hydrocarbon gases roiling beneath, he stuck a tube deep inside each vent, collecting emissions in a steel canister for later analysis in a laboratory at the University of California at Irvine.

Hower also retrieved a weathered contraption perched at the entrance to one of the vents. Cobbled together from galvanized-steel stovepipes and heat-resistant tape, this assemblage, nicknamed the Tin Man, had been taking measurements for 22 days. Three layers of filters impregnated with activated carbon captured mercury emissions, and a pair of instruments recorded temperature and carbon monoxide every 10 seconds for three days. Another set of devices monitored the same parameters every minute for the entire duration. Through these measurements, the team will gain a better understanding of the long-term variation in the fire’s temperature and emissions.

This was the second Tin Man. The first, deployed during a 2009 study, showed that the carbon monoxide level at Ruth Mullins dropped dramatically once a day and then shot back up again. “These mine fires seem to have a regular breathing cycle,” Hower says.

Coal fires are as ancient and as widely distributed as coal itself. People have reported fires in coal beds close to the earth’s surface for thousands of years—in fact, Australia’s Burning Mountain, once thought to be a volcano, sits atop a coal seam that has been on fire for some six millennia. But ever since the Industrial Revolution, the number of coal fires has grown dramatically. There are now thousands of such fires around the world, in every country—from France to South Africa to Borneo to China—where mining exposes coal deposits.

These fires are an insidious, persistent, and often nearly invisible threat to local health and to the natural and built environment. Added to that, there is now a growing realization that all these coal fires together may contribute significantly to climate change, a risk that has inspired the United States Geological Survey (USGS) to measure emissions of greenhouse gases (pdf) and other pollutants from coal fires around the United States, starting with three in the Powder River Basin of Wyoming. The USGS effort, including scientists from organizations around the country, was convened to employ new tools and expertise to measure greenhouse gases from coal fires, which have not been included in previous national and worldwide surveys. “What is the overall contribution of these coal fires to global warming?” asks Glenn Stracher, a geologist at East Georgia College whose work inspired the USGS effort. “That’s an important question that no one has answered, and that’s why this team of scientists has gotten together to work on a quantitative analysis.”

Most Americans are unaware of these long-burning coal fires, with the possible exception of the mine fire in Centralia, Pennsylvania. In 1962 residents of this small mining town burned trash in an abandoned strip mine used as a dump near the Odd Fellows Cemetery, not realizing that the mine had not been properly sealed. The trash was reduced to smoldering piles, which firefighters later extinguished—or so they thought. But the fire continued to burn, and a month later bulldozers arrived for a more concerted effort to put it out. The citizens then discovered that the dump contained a 15-foot-long opening that connected to a maze of underground mine tunnels. These passages allowed the fire to spread to the coal seam underneath the town and expand along four fronts, eventually affecting a surface area about two miles long and three-quarters of a mile wide.

Since then, around $4 million has been spent to put the Centralia fire out, to no avail. It continues to burn today, moving through a vast network of abandoned mines that are still littered and lined with coal. No one knows how extensive these empty spaces are, and the effort to quell the blaze has come to an end. “It’s too expensive to tackle, and we’re not sure we can do it anyway,” says Alfred Whitehouse, chief of the Reclamation Support Division of the federal Office of Surface Mining.

The town of Centralia is almost completely deserted today. After some residents passed out from carbon monoxide inhalation and another fell into the earth in 1981, when the ground suddenly collapsed—as the coal burns away, the ground above it often subsides into the resulting cavity —Pennsylvania received $42 million from Congress to relocate Centralia’s residents. Folks accepted the buyout one by one, and their homes were demolished to discourage squatters. (Nine holdouts are still fighting eviction today.) The town now looks like a giant vacant parking lot. A few intersections still sport stop signs, which spray painters have modified to read “Don’t STOP believing.” Aside from the eerie emptiness, signs of the fire below are subtle. On a day in January, dead grasses bristle with ice along the edges of long cracks in the earth, and wisps of gas drift here and there. An area the size of a small house recently sank about three feet, and a bright green band of vegetation flourishes in the steaming, broken earth around it.

Scientists trek up Lost Mountain to explore the underground Ruth Mullins coal fire in Bulan, Kentucky. Glen B. Stracher

When Stracher first visited Centralia in 1991, the town looked even more like a disaster zone. Stracher had just finished his postdoctoral training in metamorphic petrology; as a new professor at Bloomsburg University of Pennsylvania, he went to Centralia on a geology field trip. He was horrified by the sinkholes encased in sulfur and other precipitated minerals, the huge cracks in now-abandoned Highway 61 near town, the thick fumes rising from a ravine called Death Valley, and the sulfur-laden trees around the ravine. The town’s Catholic church was still standing then. Stracher posed for a photograph next to a mournful sign outside the church that read, “Centralia: Coal mine fire is our future.”

“The fire had been burning so long by then,” he recalls. “I wondered what long-term effect it was having on the atmosphere and groundwater, even on people who didn’t live there.”

At that point, Stracher did not know very much about coal, but he had a strong background in chemical thermodynamics. He decided to study the behavior of the sulfur coming out of Centralia’s burning coal. In 1995 he reported that some of the sulfur crystallized and stayed on the ground, potentially tainting the local water, and some of it floated away as a gas, polluting the air. Nine years later, he and a former student published an article in the International Journal of Coal Geology titled “Coal Fires Burning Out of Control Around the World: Thermodynamic Recipe for Environmental Catastrophe.” Over the next few years, Stracher was asked to put together symposia, one for the American Association for the Advancement of Science and another for the Geological Society of America. By that time, coal fires had become his life work.

After that first trip, Stracher quickly learned that Centralia was not America’s oldest or biggest coal fire. It was not even Pennsylvania’s oldest or biggest coal fire. At last count, the United States had 112 documented underground fires like Centralia and Ruth Mullins, along with many more yet to be counted. In addition to the underground fires, there are also 93 known surface coal fires, some of them in huge waste piles created during the process of coal mining. Stracher mentions a 100-foot-high burning “gob pile” (containing pieces of coal mixed with mudstone) near Birmingham, Alabama. The pile caught fire 20 years ago and was apparently extinguished at the time. But it reignited in 2006, emitting large amounts of smoke and toxic gases that caused respiratory complaints; the effort to extinguish it for good was just completed last March. Other surface fires occur where coal seams that sit close to the earth’s crust are ignited by lightning strikes, forest fires, or brushfires. The coal-rich American West has a long history of such fires—in fact, the Powder River, whose basin in northeast Wyoming and southeast Montana is the source of about 40 percent of America’s coal, was so named because the area smelled like burning gunpowder.

But America’s problem with coal fires is small compared with that of the rest of the world, where untold thousands of coal fires—no one can come up with a number judged to be even remotely accurate—burn unchecked. Eastern India has the densest concentration of coal fires in the world. Sixty-eight of them burn within a 174-square-mile region in the Jharia Coalfield in the state of Jharkand, some right next to areas where mining families live.

In China, estimates of the amount of coal consumed or made inaccessible by uncontrolled fires runs as high as 200 million metric tons per year, 10 percent of the country’s total coal production. Indonesia, a major exporter of coal to the Pacific Rim, has many thousands of coal fires. Whitehouse spent several years there fighting the burn. In a 2004 paper, he stated that the number of coal fires in eastern Borneo might be as high as 3,000. Today he thinks even that estimate was far too low. “The real number is so astronomical that no one would believe it,” he says. “The published numbers are about one percent of what could actually be there.”

Most of the coal fires in Borneo start when local farmers and plantation owners burn brush to clear land for planting, accidentally igniting a coal seam just under the surface. Fires in both abandoned mines and waste piles sometimes start because of a nearby blaze, but they can also ignite through spontaneous combustion: Certain minerals in the coal, such as sulfides and pyrites, can oxidize and in the process generate enough heat to cause a fire.

Given the implications for safety, health, and climate, the paltry attention paid to coal fires puzzles and angers many. “Most of our efforts are unfunded or funded with a shoestring budget,” says Anupma Prakash, a geologist at the University of Alaska with a career-long interest in coal fires. She, Stracher, and Ellina Sokol of the Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, are coeditors of Elsevier’s four-volume Coal and Peat Fires: A Global Perspective, the first volume of which will be published this year. Even other scientists can find the issue obscure. “People ask me why they should worry about coal fires and want me to give them some numbers and hard facts, but the reliable quantitative data are not there at the moment,” Prakash admits.

The USGS team wants to plug that gap, deploying ground sensors to tally the surface carbon dioxide emissions from a coal fire and then comparing the measurements with those from aerial surveys of the same fire using an infrared camera. By calculating the amount of burning coal needed to produce the hot spots picked up by the infrared, scientists can determine the amount of carbon dioxide such a fire should release. If both methods yield comparable measurements, the researchers will know they are closing in on solid data.

Last year the team spent three days clambering around the Powder River Basin, measuring gases from 29 vents at three fires. This alone would have given an incomplete assessment of emissions, because coal fires also release gases through the soil. So USGS geochemist Mark Engle built an “accumulation chamber” that measured gases coming out of the ground along a 119-point grid. He found that even in places where burning coal was so deep in the ground that there was no visual evidence on the surface, there were still significant amounts of carbon dioxide rising up. In fact, nearly as much CO2 entered the atmosphere through the soil as from the vents. “The gas diffused out of the soil is not real obvious,” Engle says. “The ground is not necessarily hot, and you can’t trust the vegetation to tell you what’s going on.”

While the ground crew worked during the day, the airborne crew took off before sunrise; a cold, dark night provides the best contrast between the coal fires and the surrounding land. In the final analysis, the ground-based and airborne assessments of carbon dioxide agreed within one order of magnitude, close enough for the project to be considered a success. By showing that remote sensing not only can find coal fires but can accurately assess the amount of carbon dioxide and other gases being emitted, the USGS team has introduced an easier way of assigning solid numbers to the greenhouse gases emanating from coal fires around the world.

The team has yet to release its final numbers from the Wyoming study, but Hower and O’Keefe’s earlier studies of the Ruth Mullins fire provide some sense of scale. Although the carbon contribution from each fire may seem modest, their prevalence and longevity add up. “They might not put out as much as a coal-burning power plant,” O’Keefe says, “but they have usually been burning a lot longer and will keep burning a lot longer.”

Coal fires also release a broader palette of noxious pollutants. When coal is burned in a power plant, operators make sure the fire gets plenty of oxygen so that it burns hot enough to produce the most possible energy and the fewest by-products. Coal burning in an abandoned mine typically gets far less oxygen. As a result, the coal smolders and releases a wide range of nasty, partially oxidized compounds. Testing at Centralia has revealed 45 organic and inorganic chemicals, including toxins like benzene, toluene, and xylene. Fifty-six compounds have been identified in the gases from one of China’s coal fires.

“As the gases come to the surface, they react with rocks along the way and the chemistry is constantly changing,” Stracher says. “It’s very complex. We’ll get a sample analyzed, and the chemist will say there are 40 to 60 compounds in it, and we have no idea what chemical reactions produced these compounds.”

Depending on variables of chemistry, population, and ecology, health effects may be profound. According to O’Keefe, the Ruth Mullins fire presents a respiratory health hazard to the Route 80 area of Kentucky, while the state’s Laura Campbell fire threatens the water supply. The gob fire in Alabama was the cause of traffic accidents. The Jharia Coalfield fire in India is responsible for cases of asthma, chronic bronchitis, and lung and skin cancer. One study suggests that coal fires in the United States may spit out as much as 11.5 metric tons of mercury annually, nearly a quarter as much as all the nation’s coal-fired power plants. And unlike power plant emissions, coal fire emissions cannot be regulated or controlled.

Many of the worst coal fires are in remote areas or in poor regions where ruined communities and impaired public health have not commanded much attention. Even in Centralia, the groundwater potentially tainted by the town’s long-burning coal fire has never been tested. But with a growing recognition that coal-fire emissions may threaten the planet as a whole, scientists and others hope more resources will be deployed to put the insidious fires out—a task that is much harder than it sounds.

Extinguishing a fire requires cooling the coal and isolating it from both the heat and the oxygen that feeds combustion. Surface fires are the easiest to put out, with firefighters creating moats or breaks to keep the fire from spreading and then smothering it in a nonflammable material, most often clay. Slightly deeper fires can sometimes be quenched by digging out the burning coal—in Indonesia, Whitehouse’s crews did this by hand—and then burying the entire area. But fires that rage deep underground, fed by oxygen coming from cracks in the earth, are extremely difficult to deal with. Most solutions involve pumping some combination of mud or fly ash combined with an inert gas or water, but the mixture does not always flow thoroughly enough to cover the burning coal, and it can crack when dried, allowing oxygen to get back in.

Stracher believes that one of the most promising approaches has been developed by a veteran Texas firefighter named Mark Cummins. Back in the 1980s Cummins developed an improved system to produce fire-fighting foam, and he has a long history of working with the material. In 2003 he used his nitrogen foam (made by condensing a laundry-detergent-like material) to act as a blanket, separating fire from the oxygen that fed it to help extinguish a blaze in West Virginia’s Pinnacle Mine. Since then, Cummins has developed a foam that is loaded with microbes. After he puts out a fire with his original nitrogen foam, his plan is to shoot this second foam into the mine, where the microbes will consume oxygen and replace it with carbon dioxide. “At that point, you couldn’t burn that mine if you set off a bomb down there,” he says.

Up to now, interest in such expensive, little-tried approaches has been low. But Hower hopes that an expanded understanding of the environmental impact of these fires will change things. He notes that the Ruth Mullins fire is migrating slowly toward nearby Highway 80. If a coal seam burns through the road, asphalt could crack open and sink, swallowing people and cars and unleashing a hellish scenario that might finally make people pay attention to what is going on beneath their feet.