The Bureau of Mines conducted a field investigation of the water quality in inundated underground coal mines in the northern anthracite field, eastern Pennsylvania. Water samples were collected at multiple depths from nine abandoned mine shafts in the Wyoming Basin, ranging from 68 to 650 m (223 to 2,132 ft) in depth and intersecting as many as seven mined seams in a single shaft. The shafts were also monitored for fluid resistivity, fluid temperature, eh, and ph using downhole instrumentation. The monitoring program showed that the formerly highly acidic mine water is now slightly alkaline. The monitoring program also revealed significant vertical stratification in water quality in five of the nine shafts, each characterized by two easily recognizable zones. The transitions between zones were sharp, marked by rapid changes in eh, ph, and ionic composition, the upper zones being less contaminated than the lower zones.