The synclinal basin of thePittsburgh coal (Upper Pennsylvanian Monongahela Group) is one of the oldest and largest districts of the Eastern coal fields. It contains some of the largest underground coal mines in North America, many of which produce acid drainage upon closure. Today reserves have been 50-60% extracted, and while mining continues deep along the basin axis, shallow mines along the 220-km perimeter of the syncline are separated and hydraulically isolated from the deeper mines by thick barriers. Nearly all the shallow mines are closed or near closure, including many in the period 1987-2002. Protective pumping for active operations has largely ceased and regional up-dip flooding is well underway. Saturated mine areas have been identified based on monitoring well data, groundwater flow modeling, and GIS mapping to create a map history of flooding progress at decadal scale (1980-2002). The timescale for flooding is on the order of a decade or less for individual mines but depends on local infiltration rate, rates of barrier leakage between mines, and water management by active companies. Mines closed post-SMCRA (1977) are being controlled close to, but below, surface drainage by pumping of several operators, with treatment prior to discharge. However, modeling indicates isolated locations where there are no responsible parties and flooding may proceed to the surface without control unless intervention occurs. The reconstructed hydrogeology of these flooded and flooding mines is being applied to develop solution options to prevent large-scale environmental discharge of metals and acidity.