"These are the most scrutinized and most engineered earthen structures in the world, certainly in this country," he said. "They're monitored routinely. They have lots of eyes looking at them. ... Anytime there's a heavy rainfall, the agencies are out there looking at them."Pennsylvania-based Consol was working to raise the elevation of the impoundment when the accident happened, vice president for safety Lou Barletta said. Once the worker is found, the company will determine what happened "so we can learn from it and prevent similar incidents from occurring in the future," Barletta said. The pond encompasses about 78 acres and is estimated to hold at least 1.6 billion gallons of wastewater, the equivalent of more than 2,500 Olympic-sized swimming pools. Impoundments typically hold up over time. But they do fail, and in spectacular fashion. In 1972, an earthen dam in Buffalo Creek in southern West Virginia collapsed after heavy rain, unleashing 130 million gallons of water, sludge and debris that killed 125 people, injured 1,100 and left 4,000 homeless. Spadaro dedicated his life to preventing that sort of disaster from happening again and helped write the nearly 40-year-old regulations that are now on the books to guide slurry pond construction. The regulations give the government and coal companies detailed design, compaction strength and other criteria so the structures will withstand internal pressures and additional stress from big rain storms. The regulations require the operator to do daily, quarterly and annual inspections, while the state must make monthly checks. MSHA relies partly on data provided by the companies to spot problems, which critics see as a weakness in the system. Spadaro is not the only one to question the soundness of impoundments. Retired miner-turned-activist Joe Stanley raised concerns over the Brushy Fork impoundment in southern West Virginia last year, but his complaints were dismissed by state regulators, and their decision was upheld by the federal Office of Surface Mining Reclamation and Enforcement.