On October 11, 2000, an estimated 306 million gallons of water and fine coal refuse slurry broke through a bedrock barrier from an impoundment in Martin County, eastern Kentucky, into an adjacent underground mine. Approximately 260 million gallons of the water and coal slurry discharged from two underground mine portals and affected over 75 miles of streams in Kentucky and West Virginia. As a result of this and several other breakthroughs over just half a decade, the U.S. Office of Surface Mining Reclamation and Enforcement (OSM) and other institutions undertook investigations to assess the causes of the events, the potential for additional breakthroughs in the future, and available methods for preventing them. In addition to needed improvements in the design, construction, and inspection of the facilities, the studies have addressed issues pertaining to the flow characteristics of refuse slurry, not only in impoundments still receiving pumped slurry, but also in “idle” and reclaimed facilities. Related questions concern: (1) the effects on breakthrough potential of the impoundment abandonment process and construction of slurry cells on top of capped structures; and (2) appropriate measures and available methods that may be used to ensure that underground mines adjacent to or underlying impoundments are known and accurately located. Current information on the engineering properties of coal refuse in existing facilities provides no assurance against fine refuse flowability during any stage in the impoundment construction and reclamation process or after reclamation has been completed. Due to this uncertainty, thorough site investigations and conservative measures in design, construction, reclamation, and quality control are of paramount importance.