[baxinho0312]

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

[thaumaturgy]

Two more issues are water tables (although those have been falling rapidly across much of California over the last few years) and seismic activity -- not just the big quake sort, but also the usual few cm a year of movement in opposite directions sort.

I suspect that this is going to be an engineering project on the scale of the mythical California high-speed rail, and with the same life cycle. PG&E especially has a track record of misusing funds for upgrades and maintenance.