The cause and mechanism of a sinkhole that has occurred on the downstream shoulder of a large embankment dam are studied. Since the downstream shoulder of the embankment was softened as a result of saturation, the slope safety of the embankment dam was reanalyzed by extending the saturation zone beyond the filter layers. Calculation results show that the dam is not safe currently in the area of the sinkhole section. The source of this problem was sought. Excavation of a trench at the sinkhole section was planned to take immediate action against failure. This trench excavation resulted in a fast, inexpensive, unambiguous, and effective way to rapidly identify the reason for the sinkhole. Trench excavation showed that the sinkhole problem was associated with the deterioration of the curved trace of the bottom outlet conduit: a rupture in a steel conduit at a poorly welded joint and a poor concrete section with a lack of reinforcement. High reservoir pressure through the rupture caused particle rearrangements, heave occurrence, upward propagation of cavities, and finally a sinkhole. In the study, remedial measures and the lessons learned from the case are also explained. (C) 2016 American Society of Civil Engineers.