Failure of the seepage barrier between a coal waste impoundment and an underlying underground mine caused millions of cubic feet of black slurry to enter the mine and flow out into area streams in late 2000. Extensive investigations were made to determine the causes of the failure. Government-sponsored studies reported that the failure was due to piping, but no seepage analyses were reported to support this conclusion. Consequently, a comprehensive program of analyses was undertaken. Those analyses show that, in all probability, piping did not cause the breakthrough. This result is important because controversy surrounds this failure, and government reports are directing future remedial measures for similar structures away from the most practical solution to the problem.