The underground below the hill has been crisscrossed by a network of underground corridors or "lagums", dating back from the period of Austrian rule in Zemun. They connect Ćukovac, Muhar, Kalvarija and Gradoš. This terrain is one of the most active landslide areas in Belgrade. Being cut into for centuries, the loess in some sections have cliffs vertical up to 90%. In the previous centuries, settlers left many vertical shafts which ventilated the lagums, drying the loess and keeping it compact. After World War II, inhabitants began to settle Ćukovac hill unaware of the lagums, especially the largest one, which covered an area of . As there was no sufficient sewage system at that time, they built septic tanks and collected rainwater, but also as the ventilation shafts in time were covered or filled with garbage, it all made the ground wet in the course of several decades. The lagums retained the moist and began to collapse. Eventually, the walls and houses became unstable to the point of breaking façades and walls. In 1988 city authorities finally intervened as the houses began to sink in three streets. Holes were drilled to connect the surface with the largest lagum. Altogether, 22 drillings were made and of concrete were poured into the lagum, filling it until the ground was stabilized, but the lagum was destroyed in the process. Still, the situation is critical after almost every downpour. Though it can be built on loess, in contact with water it turns into the sand which causes all the problems. It has been suggested that a better, natural way of draining the loess is the planting the groves of Atlas cedar. It is also much cheaper than the expensive construction projects as this type of cedar has a deep and very developed roots, which drains all the atmospheric precipitations, leaving the loess dry. Additionally, Atlas cedar grows very fast in Belgrade climate, while the tree itself is an appealing park species.