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.
[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/
There are some limitations; for instance, on long transfer lines underground - we're talking about 100 km here - you may need to convert to DC in between because underground cables have stronger reactive components which makes voltage control difficult.
The pylon shown is the end one — the wires on the left side go down to the ground. It's probably on the edge of a city somewhere, within British cities electricity distribution is buried.