Wow $3M a mile to bury lines is insane. It sounds like that’s the only solution though?
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It's not impossible to do things, but people (in the US) are extremely resistant to any kind of perceived change or self sacrifice.
It's a common story for any kind of infrastructure project in the US.
Overhead distribution wires are insulated primarily by air and distance; wires in the ground need more material and cost to be insulated from the ground and each other. That insulation and the ground is likely to trap heat; in urban settings, it's common to have fluid filled underground cables to manage heat, but I don't know how necessary/viable that is in rural areas.
Construction is more expensive and invasive. Rather than clearing to ground level every so often for towers and then managing (or not) tall trees between towers, you've got to clear to ground level for trenching. I'm pretty sure you can't lay distribution lines with directional drilling like you can with communications cables; you've also got to do a lot of specialized fill around the cables themselves.
Maintenance is more expensive; you need to keep the ground above the cable clear, so that it's possible to access when needed, and so tree roots don't mess with your cable.
Repairs are more expensive and time consuming: damage could be hidden, and anyway you need to dig to repair breaks or replace sections of wires. This one trades off with the expectation that underground wires will require fewer repairs.
Connecting is more expensive; underground utilities may require digging to access.
They tend to keep the ground cleared under high voltage transmission lines too.