The most toxic byproduct of coal-fired power isn’t carbon dioxide. It’s the residue that’s left over. When coal is burned in a plant, just like in a home, it produces two kinds of ash: fly ash, which rises, and bottom ash, which sinks to the floor of the furnace. Together, they comprise coal ash. In a power plant, all that ash has to be put somewhere.

For many years Kingston was the largest coal plant in the United States, which meant the facility produced a lot of coal ash. It was mixed with water to stop it from blowing around and dumped into the retaining pond, which quickly swelled with the waste. As the region grew, demand for electricity soared, more coal was burned, and the TVA built the walls of the pond higher and higher. By 2008, they stood 60 feet tall, and the pond held more than a billion gallons of coal ash slurry.

Shortly before 1 A.M. on December 22, 2008, the walls broke.

One corner of the dike, exhausted after a half-century of pressure and decay, collapsed. The coal ash sludge poured out, tearing a hole that kept on growing. Once outside, the slurry built up force and barreled over a sequence of outer dikes.

With no barriers left, the sludge made its way 200 feet down to the banks of the Emory. It was so strong and dense that when it hit the water, it forced the river to change its path, pushing it eastward. The water, glistening just hours before, became a thick, gray soup rich in carcinogens and heavy metals.

For about an hour, while the town of Kingston slept, the slime spilled further and further from its retaining pond. The earth shook. The spill became a conveyer belt that slowly moved through the surrounding forest, ripping trees from their roots and driving them along its path. A cell-phone tower was thrown to one side. A railroad line was buried. The sludge picked up a bulldozer and swept it nearly a thousand feet.

In fact, there was so much ash that it pushed upstream as well as down, moving along the Emory and into the nearby Clinch River. Nearby properties came into its range, and it began climbing up into the backyards and patios of houses along the Clinch’s banks.

Three houses were destroyed, and another 42 properties damaged. No one was hurt, but Kingston residents said that was a stroke of pure luck. “Just about any other time, other than the very middle of the night of the longest night of the year — this was the winter solstice — we’d have had a body count,” says local resident Steve Scarborough.

The TVA didn’t learn about the disaster until emergency radios started carrying the message of a 911 caller. A 45-year-old firefighter had been woken by the noise, gone outside to figure out the cause, and called in a problem — but what was it? A landslide? He couldn’t tell. Then he saw that a nearby house had moved across the street; his neighbor had had to break the bedroom window and climb out to escape his house as the sludge destroyed it.

A TVA employee heard over the scanner that there was a problem at Kingston, and called the foreman of the plant’s ash storage system. Not long after, TVA police called Tom Kilgore, the company’s CEO, and roused him from bed.

Coal remains the dominant source of electrical power in America, responsible for just under 40 percent of the nation’s supply. The TVA is the largest public power provider in the country, serving nine million people across seven states.

Like many other power companies, it has slowly been expanding its energy portfolio, but coal remains its backbone. And, like the rest, it has been upgrading that infrastructure, forced by the Environmental Protection Agency to make its plants run cleaner and more efficiently. Usually, that means installing scrubber systems in the smokestacks, which clean the emissions of pollutants like mercury, toxic metals, and acid-rain-causing sulfur dioxide.

The result is that the exhaust that emerges today is far cleaner than it was decades ago. But cleaning up the airborne emissions means that the solids remaining after the burn are far dirtier than before. In fact, coal ash is often loaded with arsenic, mercury, lead, and other contaminating elements. A 2012 study published in the journal Environmental Science & Technology found that cleaner air emissions are traded for “significant enrichments of contaminants in solid wastes and wastewater discharged from power plants.”

According to the American Coal Ash Association, the nation churns out nearly 65 million tons of coal ash annually. However, despite the fact that it is replete with toxins, coal ash is essentially unregulated by the federal government, with oversight meant to happen at state level. Yet most states handle the material with less precaution than your standard household garbage.

The “best” use for coal ash, say scientists, is to recycle it and use it in cement, which means the contaminants get locked in and cannot leach out. But there are a range of other recycling methods, including using it in asphalt and wallboard, spreading it as a soil amendment, or using it as a substitute for salt on icy roads. One particularly novel method of disposal is dumping it into abandoned mines. Most coal ash, though, is simply carted off to landfills or placed in retaining ponds like the one at Kingston. Many of the ponds, including Kingston’s failed one, are not just built to contain the coal ash, but actually use it as a construction material in their dams and dikes.

Kingston TVA, Harriman, Tennessee. Coal ash cleanup effort. © J Henry Fair, Flight provided by Southwings www.southwings.org

In Kingston, it was the dike that broke, but spills and contamination can happen in all kinds of ways. In 2005, a log wall broke in a basin at a Pennsylvania power plant, spilling at least 100 million gallons of coal ash into the Oughoughton Creek and Delaware River. In October 2011, a bluff at a Wisconsin power plant collapsed, spilling coal ash into Lake Michigan.

Environmental groups dislike pond storage, arguing that wet ash is more likely to leach contaminants into nearby water supplies, as well as being a far greater risk for catastrophic spills. A draft EPA risk assessment released in 2010 showed that coal ash ponds pose greater risk to human health due to leaching than landfills.

Charles Norris of Geo-Hydro Inc, a scientist from Denver who has testified before Congress on the subject of coal ash, says things are made even worse by using it as a construction material. Not only does this let even more contaminants leach out, but over time it compromises the structural integrity of the pond. Kingston, Norris believes, is unlikely to be the last incident of its kind. “The use of this as a construction material is new enough that we’re probably just looking at the beginning few failures,” he says.

n the Sunday evening after the spill, the TVA held a community meeting at the Roane County High School gym. Residents were concerned by the repercussions on the environment and their health, and were clamoring for an extensive cleanup. Steve Scarborough, a retired engineer who has lived in town since 1988, was distraught. “My beautiful clearwater view, the waterfowl — ducks and geese and herons and all that — went from one day being picture-perfect to the next day being a disaster,” he says.

“Everything that had been alive,” he says, “was buried under that mountain of coal ash.”

Tom Kilgore, the CEO of the TVA, turned up to listen to the worries of locals. An industry veteran, he had been appointed to run the organization after decades at other energy companies in the South. With the community jostling for action, he pledged a full cleanup could be done in weeks: “It will be a long job and a hard job, but we will get it done, and done safely and in a way that protects our environment, recovers the area, and makes it nice again.”

Within days of the spill, massive yellow construction and dredging equipment gathered in Kingston. It was a multi-pronged attack. Out in the river, hydraulic dredges dug ash from underwater and pumped it through pipelines that carried it thousands of feet to temporary storage areas. Near to shore, amphibious track hoes dug more ash and poured it into dump trucks waiting on ash-smothered makeshift roads. On land, excavation equipment loaded the ash into temporary storage piles. The spill was so big, though, that it was like trying to empty an ocean with a teaspoon.

And while Kilgore vowed to fully clean up the spill, the TVA made it hard to measure progress. Residents who tried to visit the spill site, or to paddle or boat down the river to get a closer view, were stopped by local police or TVA security.

The corporation also failed to address another significant question: What was the composition of the spill? The Environmental Integrity Project, a Washington D.C. nonprofit, decided to dig through the complex data to find the answer. It estimated that 140,000 pounds of arsenic had spilled into the Emory River, as well as huge quantities of mercury, aluminum and selenium. In fact, the single spill in Kingston released more chromium, lead, manganese, and nickel into the environment than the entire U.S. power industry spilled in 2007.

The dangers of all those chemicals are well known. Arsenic is associated with bladder and liver cancer. Lead causes damage to the brain, kidneys, nervous system and red blood cells. Mercury causes brain and kidney damage. Aluminum can compromise lung function. Hexavalent chromium is the “Erin Brockovich chemical,” and is known to cause cancer and damage to the respiratory system, kidneys, and other organs.

Dennis Lemly, a U.S. Forest Service researcher, has been studying selenium in coal ash for decades, and warning public agencies about the dangers of coal ash pollution the entire time. In the 1970s, he studied Belews Lake in North Carolina. The lake became so contaminated with selenium from coal ash that ultimately all but one species of fish was wiped out. Lemly has photos of fish from that period with spines curved so badly they looked like slithering snakes; fish with deformed mouths, which limited their chances of survival because they couldn’t eat very efficiently; and “popeye” fish with bulging, cartoon-like eyes.

Belews Lake was supposed to be a worst-case scenario. Yet Lemly recently released a new report documenting many of the same deformities in another North Carolina lake that receives coal ash waste from a nearby power plant. “The truth is, there’s a lot of [coal ash–damaged ecosystems] out there,” he says.

Kingston, though, is by far the worst coal ash disaster that the industry has ever seen: 5.4 million cubic yards of coal ash, containing at least 10 known toxins, were spilled. In fact, the event that woke Sarah McCoin that night — the deluge that moved houses and ripped trees from the ground — was even bigger than the Deepwater Horizon oil spill in April 2010, which spewed approximately 1 million cubic yards of oil into the Gulf of Mexico.