In a part of the city of Zagreb built on southern hill-sides of the Medvednica mountain one is in the day-to-day geotechnical practice often confronted with the problem of soil instability (slide and creep). As usual, causes of the ground destabilization are various (geological and hydrogeological predispositions and influence of man). At certain of investigated sites existence of underground interfaces with specific characteristics was found. As it was assessed that their presence can have an essential influence on the terrain stability, a programme of investigating such phenomena was prepared. The paper presents results obtained so far by investigations performed at two sites.