Placing transmission and sub-transmission voltage power lines underground is significantly more expensive than placing power lines overhead. Actual cost differences depend on various elements, including terrain, project length, environmental concerns, labor, and material differences, etc. An overhead line that spans trenches, washes, or other difficult terrain, may not be compatible for running underground, and would require added time and expense for additional labor and material. Depending on the voltage and the location of the power line and considering the inability to be cooled by the ambient (surrounding) air as in an overhead configuration, underground power lines may require special technology to keep the wires cool. Underground 230kV lines require the cable conduits to be encased in a special concrete slurry, which allows for greater heat dissipation while adding a layer of protection from accidental excavation.
Buried power lines may also extend power outages, as it may take additional time to locate a specific power issue, access the fault and repair it. Additionally, underground systems can be prone to flooding in certain conditions. Typically, lower voltage 12kV (distribution) lines are buried with new developments, as the additional cost to place the lines.