It is physically impossible to have long distance underground high voltage AC transmission lines due to capacitance: http://www.puc.nh.gov/2008IceStorm/ST&E%20P

It is physically impossible to have long distance underground high voltage AC transmission lines due to capacitance: http://www.puc.nh.gov/2008IceStorm/ST&E%20Presentations/NEI%20Underground%20Presentation%2006-09-09.pdf http://www.puc.nh.gov/2008IceStorm/ST&E%20Presentations/NEI%... HVDC wouldn't have that issue, but there's not much experience with HVDC in North America.