The residue of millions of tons of coal burning at Kingston Fossil power plant in the Watts Bar Reservoir in Tennessee burst the bounds of the pond in which it was contained, burying as many as 400 acres of land in up to six feet of sludge. The Tennessee Valley Authority (TVA), which owns the coal-fired power plant—first operated in 1955—announced that 15 homes were buried and no injuries were reported.

A combination of rains and accumulating sludge likely contributed to the disaster—one of two major ash pond collapses in the past decade. All told, about 2.6 million cubic yards of so-called coal ash slurry escaped, the TVA says. The collapsed pond is one of three on the site.

"We deeply regret that a retention wall for ash containment at our Kingston Fossil Plant failed, resulting in an ash slide," said Tom Kilgore, TVA president and CEO in an official statement today.

Such slurry worries environmentalists and public health activists because it is the residue of coal burning. The burning concentrates the impurities in the coal, including arsenic, lead and mercury, among many other potentially toxic contaminants. Coal ash is also radioactive.

But dealing with the 129 million tons of coal ash produced in the U.S. every year is not easy. Some 25 million tons of it is dumped in old coal mines, and some companies incorporate it into cement. The rest is typically dumped in landfills or stored in large coal-ash ponds like the one that collapsed. But many environmentalists argue for only disposing of it in lined landfills, to prevent contaminants from leaching out.

"A risk assessment released by the U.S. EPA [Environmental Protection Agency] revealed that coal ash poses extremely serious threats to human health and the environment when disposed in waste ponds and landfills," says Lisa Graves Marcucci, a founding member of the Jefferson Action Group, a Pennsylvania environmental group that is among 38 environmental organizations calling for the incoming Obama administration to review coal ash disposal rules. "Significant pollution from mine disposal has been documented in New Mexico, West Virginia, Indiana, North Dakota, as well as Pennsylvania."