Failure of Lone Pine Reservoir to hold water admirably illustrates the result of complete disregard of geologic conditions. The earthfill dam, 101 feet high, built in 1934--36, proved satisfactory, but the reservoir developed serious leakage almost immediately and lost nearly half its stored capacity within one month. Silt-filled sinkholes, fissures, and openings were breached and an integrated ground-water system established by the pressure head of 74 feet of water in the lake. All attempts to "seal" the reservoir have failed.Rocks of the area consist of: an undifferentiated series of Permian Coconino-Kaibab beds; thin Triassic sediments; both the volcanic (?) member (Lower basalt) and the upper member (Gravels) of the Pliocene Bidahochi formation, and locally capped by the Quaternary (Upper) basalt. The reservoir is largely in the Coconino-Kaibab sandstones and limestones; salt beds possibly occur beneath this series. The dam abutments are of well-jointed Lower basalt that overlies an irregular limestone unit. A regional, gently northwest dipping homocline is interrupted along the west edge of the proposed reservoir basin by a subsidiary, north-south anticline.This study considers, first, the cause of the reservoir failure and second, the feasibility of so sealing its basin as to render it usable. All rocks involved permit leakage; the jointed basalt with interbeds of ash; the inherently permeable Coconino-Kaibab beds; their secondary fractures and dissolved condition of sinkholes and interconnected openings; and the possible occurrence of salt beds at depth. The regional structure and a deep static water level provide a natural "piping circuit."Reservoir sealing techniques, discussed in some detail, offer no sound economical solution for repair of Lone Pine Reservoir. Satisfactory undeveloped reservoir sites of similar capacity, available a few miles upstream, can be utilized for a cost similar to that of an attempted patching of Lone Pine Reservoir.