Burying high voltage power lines presents several technical challenges on its own - the type and thickness of the insulation had to be able to withstand the dielectric stress which is normally solved by using enough distance between the wires themselves and the ground for the typical high voltage power line. Another problem is increased capacitive load due to the higher capacitance of the lines which causes problems/completely makes the switchgear to not be able to perform switching/breaking operations. Capacitive load then needs to be compensated to improve the networks power factor, cos phi. This is typically done by introducing expensive equipment to the network configuration like shunt reactors or static synchronous compensators. It is also typically more expensive to have and maintain a underground cable than an overhead power line