The Bridgeton Landfill, about 20 miles northwest of St. Louis, is in many ways a typical pile of trash. Bridgeton is a layer cake of garbage and dirt at the bottom of an old limestone quarry, all of it covered with a frosting of clay, plastic liner, soil and grass. But for the last six years, there’s been something wrong at the core of Bridgeton — a wrongness that has led to lawsuits, angry neighborhood activists and national media attention. It’s confusing and scientifically strange — and all those problems are exacerbated by the nearby presence of a big old pile of nuclear waste.

Down beneath the layers of trash bags, banana peels, Chinese takeout cartons, diapers and dirt, the Bridgeton Landfill has become very hot. Normally you’d expect the process of decomposition to heat the interior of a landfill to around 140 degrees Fahrenheit. Parts of the Bridgeton landfill, in contrast, have reached temperatures as high as 260. That 120 degrees is the difference between a healthy landfill, decomposing merrily along, and one in which the systems of safe waste management are falling apart.

Whenever a landfill develops a hot spot deep down, it’s a problem for people who live nearby, for the surrounding environment and for the operator of the landfill. High temperatures damage the plastic liners and the pipes that keep trash-infused water called leachate from seeping into local groundwater. High temperatures decompose trash differently and result in leachate that’s harder to treat and make safe, even when it is being properly captured. And high temperatures release dangerous (and malodorous) gases that cooler trash doesn’t. Those gases can find their way out of the landfill more easily because the hot spots tend to collapse, leaving cracks in the layer cake’s smooth surface.

And then there’s the bit about the radioactive waste. Bridgeton is not the only landfill with a hot spot, but it is the only one with a hot spot that’s around 1,200 feet away from about 8,700 tons of radioactive barium sulfate — a byproduct of uranium processing. It came from a factory in St. Louis that produced uranium for the first self-sustained nuclear chain reaction. That material, mixed with dirt, is part of the layers that make up the nearby West Lake Landfill. The Environmental Protection Agency, which manages West Lake as a Superfund site, believes that if the radioactive waste becomes hot, it could release cancer-causing radon gas into surrounding neighborhoods. Suffice it to say there are many reasons people want Bridgeton Landfill to cool down.

Unfortunately — despite an April 28 announcement of an agreement between the EPA and Republic Services, the company that owns Bridgeton — that’s not going to be easy. Bridgeton may be a typical pile of trash, but this is no typical trash fire. The heat exists 40 to 140 feet below the surface, in places where Republic Services believes no oxygen is present. It exists in places that are wet, soaked with leachate. Those are not conditions where fire should exist, by most common-sense standards.

There are about 8,300 landfill fires every year and they happen most frequently in summer, when people are more likely to throw out stuff like still-warm coals from a barbecue grill. But almost all of those are obvious fires. There are flames. They sit on the surface of the trash or just below it. Only 5 percent of landfill fires happen underground, according to a 2001 report by the U.S. Fire Administration.

It’s the underground fires that are the weird ones. Despite the USFA estimate of 400 or so per year, no agency is keeping track of how many there are. And some may not even be fires at all. I’ve used the term “hot spot” to refer to Bridgeton because engineers from Republic Services are adamant that it isn’t a fire — they call it a subsurface reaction — and believe it represents an unstudied form of heat-producing chemical reaction whose cause remains a mystery. Even if they’re wrong and the Bridgeton hot spot is indeed a fire, it’s a type of flameless fire — subterranean smoldering combustion — that is notoriously difficult to put out and plenty mysterious on its own. “We are talking about a completely different beast,” said Guillermo Rein, an engineering professor at Imperial College London who studies subterranean smoldering fires. “The heat-transfer processes and the chemistry that takes places in smoldering is significantly different” from flaming fires on the surface. “We know very little about smoldering. We know even less about the deep ones.”

Subterranean smoldering, like an ordinary fire, happens when heat and combustible material come together. A big difference, Rein said, is that in an underground fire there’s little oxygen to feed the reaction — which means that the smolders never burn hot enough to create flame. When Todd Thalhamer, a waste management engineer and landfill firefighter based in California, tracks smoldering fires beneath other landfills, he does it by breaking open one side of the trash pile and following the trail of plastic bags. “As you get closer to the direction of the smolder the white plastic bags will start turning a light tan, a darker tan, and eventually they’ll become a gummy black,” he said. When he reaches the heart of the smolder, the air that came in with him will sometimes cause it to burst into flames. But otherwise, what smoldering fires “look” like is a trail of hot, partially combusted material, and a low-lying haze of gas.

Because smolders burn flameless, they don’t completely combust their fuel. Instead of burning quickly to cinders, the fuel smolders, releasing a heady brew of combustible gases — which help keep the smolder going. The oldest fire on earth is a subterranean smolder, Rein told me: Australia’s Burning Mountain fire started at least 6,000 years ago and is moving through a coal seam — consuming the fuel behind it and combusting the fuel ahead — at a rate of about 3 feet per year.

Nobody has ever tried to to put out the Burning Mountain fire, Rein told me. But people have put out subterranean smoldering fires in landfills, forests and peat bogs. Unlike with flaming fires on the surface, just dousing the area in water doesn’t generally work very well, Rein said. Michael Beaudoin, an engineer who is director of remedial projects with Republic Services, agreed that water wasn’t the best way to fight a subsurface reaction — if for no other reason than the fact that those reactions can happen in material that’s already wet. Firefighters put out a smoldering underground peat fire in a forest in North Carolina by diverting local streams and completely flooding the affected area, Rein told me. But you need that kind of complete saturation, and that’s hard to achieve in a landfill.

“The amount of water you need is insane,” Rein said. In laboratory experiments, he’s found that a 1-meter cube with a smoldering fire at the bottom required 3 liters of water per kilogram of smoldering material (about 1½ quarts per pound) to extinguish the fire. That ratio is more than 50 percent larger than the water/fuel ratio needed to put out a flaming fire. And in that experiment, Rein knew exactly where the smoldering material was. “If this is not known, the amount of water required is much much larger, maybe up to 300 liters per kilogram of burning fuel,” he wrote to me in an email.

Instead, quarantine might be the best solution. “All of the experts in the academic community we’ve dealt with really say that this is something that there’s no proven way to stop,” Beaudoin told me. “Basically, it has to run out of fuel.” When a chemical reaction between water and buried aluminum dross started a heating event in the Countywide Landfill, south of Akron, Ohio, Republic Services eventually controlled it by digging a trench to separate the hot part of the landfill from the rest. This involved moving 400,000 cubic yards of trash. While Republic Services doesn’t believe the reaction in Countywide is a fire, the trench operates in the same way a firebreak would, sequestering the heat in one place where it will eventually run out of fuel. Thalhamer, the California firefighter, has used a similar technique in other landfill fires.

And at Bridgeton, Republic is trying something new. “We installed, basically, an underground radiator,” Beaudoin said. The company sank pipes into the landfill. Cool fluid is pumped down the pipes, absorbing heat from the reaction deep in the trash. When the fluid is pumped out, it takes the heat with it. After two years of testing, Republic will now expand this heat-extraction system so it forms a cooling barrier between the hot parts of Bridgeton and the radioactive waste in the West Lake landfill, aiming to keep the hot spot from spreading in that direction.

Essentially, the company plans to manage the hot spot, and wait for it to cool itself down. That’s common practice with underground reactions and fires, because managing these events is expensive — Republic is currently dealing with three heating events, including Bridgeton, and Beaudoin says each has cost the company tens of millions of dollars so far — but putting them out would be even more costly.

It’s been done. Thalhamer won’t speak about the Bridgeton case — he’s a consultant to the Missouri Department of Natural Resources, which is suing Republic Services — but he says he has put out subterranean smolders in other landfills by digging through the trash for all the hot spots and smothering them with dirt. “But it’s really expensive,” he said. “I try to stress the word ‘really’ because I don’t know what other word to use. Extraordinary? Excessive?” One subterranean fire racked up firefighting costs of half a million dollars in just five days, he said. In 2008, the Los Angeles Times estimated that one day of fighting the Zaca forest fire, near Santa Barbara, cost $2.5 million. But most landfills are owned and paid for by a single, relatively small entity — usually a county, a city, or a company. Thalhamer said that putting out a subterranean smolder in a landfill — as opposed to just managing it — could drive a company to bankruptcy.

CORRECTION (May 10, 10:17 a.m.): An earlier version of this article misstated the university where Guillermo Rein is a professor. He works at Imperial College London, not University College London.

CORRECTION (May 10, 5:50 p.m.): An earlier version of this article misidentified the material at the base of the Bridgeton Landfill. The base is made of limestone, and is not a plastic liner.