On Dec. 22, a holding wall at a waste pond at Tennessee Valley Authority’s Kingston Fossil Plant in Harriman, Tenn., collapsed, dumping more than a billion gallons of slurry over more than 300 acres, in what some are calling the largest environmental disaster in US history.

But what exactly is this stuff, and what effects will it have on the environment?

The sludge was a mixture of water and fly ash, a residue that is captured in the chimneys of coal-fired power plants. Fly ash is distinguished from bottom ash, which is removed from the bottom of the furnace.

Fly ash is mostly made of fine, hollow, glassy particles of silica, the most abundant mineral in the earth’s crust, as well as aluminum oxide, iron oxide, and lime, a white crystalline solid that humans have used for thousands of years. When airborne, some of types of silica particles have been found to be potentially harmful to people’s lungs.

But more worrisome are the trace concentrations of toxic metals – including arsenic, lead, barium, and chromium – that scientists think may damage the liver and nervous system and cause cancer. The ash also contains uranium and thorium, both radioactive elements. Ounce for ounce, fly ash delivers more radiation into the environment than shielded nuclear waste.