Oh look, a condescending asshole.
Fine. s/copper/aluminum/g Happy now?
What I said is perfectly true and applies equally well to aluminum. Heating effects are much easier to deal with underground because sagging is irrelevant and the thermal environment is very predictable. You have much more flexibility in choice of insulator because again, weight is no longer an issue.
In any case, the anonymous coward is still a babbling idiot. Power lines of all sizes are buried in every major city in the US, and they wouldn't be if there was some dramatic difference in transmission efficiency. I was and am speaking of electrical efficiency, but in fact it applies to fiscal efficiency as well. There's a couple of studies linked in this thread that demonstrate that maintenance costs for lines buried in cable tunnels is lower than for overhead wires. So much lower that it pays for the difference in installation costs over the useful life of the wires and then some.