APS has published this document to describe the technical issues with putting power lines underground.
Click here To Download: Undergrounding High Voltage Electricity Transmission Lines – The Technical Issues
The purpose of this document is to provide information about the technical merits and challenges associated with undergrounding high voltage electricity lines, compared with installing overhead lines. Overall, there are a number of issues that make the undergrounding option more technically challenging and expensive. However, despite the costs and technical challenges, there are circumstances in which underground cables are a more preferable option than overhead cables.
This document explains the cost and potential impact on the environment of cable installations. It explains the cable types available and the various installation methods, as well as the separate components that make up an underground installation.