Popular Mechanics has a good article on the subject here. Pros and cons (the cons being mostly the expense) but you’ll get a good understanding of the issues involved, I believe.
It can be somewhat affordable to use underground power cables when you’re starting from scratch; developers building new housing tracts can install buried power cables alongside fiberoptics lines and water systems.
But retrofitting is much pricier. “If you’re talking about a built environment where the lines are already up and you’d have to dig through peoples’ lawns and driveways, it becomes prohibitively expensive,” Garvin says.
For example, in a new suburban neighborhood, installing ordinary overhead power lines costs about $194,000 per mile on average. Installing underground power lines would cost $571,000 per mile. And to retrofit an older suburban neighborhood with underground lines, the costs climb up to an average of $724,000 per mile.
For high-voltage transmission lines—the thick cables typically slung between towers that carry electricity across long distances—new underground installations can cost as much as $23 million per mile. Those costs get deflected to the consumer.
Buried lines are no protection against floods, so Old Greenwich would have gone dark regardless, but the rest of the town would be sitting pretty today if our system were immune from falling trees. While I don’t see how we could afford to do this, the PM article closes with a quote from a Swiss source who points out that his country put its lines underground decades ago and wonders why on (in?) earth we don’t do the same. Probably because there’s nothing sexy about a politician standing in front of a ditch digger, but there you have it.