In the Ilirska Bistrica area the largest hitherto known landslide in Slovenia, 5 km wide and 5.5 to 6 km long, was initiated at the end of Pliocene or in the lower Quaternary. Its formation was a consequence of tectonic structure and dynamics of the movements in that area. The landslide slumped from the morphologically uplifted thrustfront of the External Dinaridic Thrust Belt into the rapidly subsiding Ilirska Bistrica basin that has pull apartcharacteristics along the Ra a fault. This structure and dynamics resulted into a failure of the supporting slope.Additional reason for triggering the slide could have been a strong earthquake in the Ilirska Bistrica seismic zone which is hypothetically connected to the Ra a fault. The entire town area of Ilirska Bistrica is situated on the toe of the fossile landslide.