The Fatha (ex-Lower Fars) Formation (Middle Miocene) is the predominant stratigraphic unit in the Mosul Dam area. It is about 250 meters thick near Mosul. Marls, chalky limestone, gypsum, anhydrite, and limestone form a layered sequence of rocks under the foundation of the dam. The foundation of the dam is mainly resting on the Fatha Formation (Middle Miocene) which is highly karstified. Karstic limestone and the development of solution cavities within the gypsum and anhydrite layers are the main geological features under the foundation of the dam. The right (west) abutment is located in the steeply dipping Fatha Formation within Butmah East anticline with SE plunge being in the reservoir north of the dam, whereas the left (east) abutment is located on gently dipping beds of the Fatha Formation, which is overlain by fine clastics of the Injana Formation. These differences in lithology as well the dip amount and direction along both abutments as well upstream and downstream of the dam have certainly affected on the hydraulic pressure and increased the dissolution ability of the gypsum and limestone beds, along the abutments and the foundations, which are already karstified in nearby areas. Consequently, more gypsum, anhydrite and limestone beds are dissolved and karst openings are continuously increasing, as the exerted hydraulic pressure is continuous.First appearance of sinkholes on the right bank down-stream was not until approximately six years after the filling of the reservoir began. The surface expression of the sinkholes suggests that they are caused by an under-ground collapse. Concentric tension cracks appear to have developed around the central void as the sinkholes have developed progressively. Karstification and formation of sinkholes are the most dangerous features threatening the safety of Mosul dam.