[arrrg]

It is possible to put that all underground, that's just extremely expensive. (That's currently a hotly debated topic in my region, since the new north-south connection in Germany is very controversial and underground transmission is sometimes brought up as a solution, though not really a realistic one.)

I really don't know if underground transmission will ever be economical. In principle it's possible, though, if you want to spend the money.

[dmethvin]

Not only expensive to run things underground, it introduces maintenance nightmares. Overhead lines can easily be inspected and even repaired while in service [1]. Underground lines cannot easily be inspected or repaired. Even when there are multiple lines, failures can overload the remaining lines as happened in Auckland [2]. If you want to get an idea of the hassle repairing an underground line is like, read this series of posts [3].

[1] https://www.youtube.com/watch?v=tA9oBEHRLqQ&hd=1

[2] https://en.wikipedia.org/wiki/1998_Auckland_power_crisis

[3] http://www.jwz.org/blog/2002/11/engineering-pornography/

[ptaipale]

Not perhaps extremely expensive, but it may be more expensive than setting up pylons. But for instance when crossing waters, it is more practical to have an underwater cable than build pylons across sea.