Above ground "High Temperature, Low Sag" cables can reach 200°C. Underground cables can still go up to 90°C, which is still flamin' hot. They're usually buried at least 1 meter deep, which gives the heat a decent chance to dissipate. But in those areas with high chances of fires, I'd be wary all the same :-/.

Of course, the above mostly applies to high voltage & high current situations, think 150kV, 300MVA monster connections. I doubt PG&E manages 10k miles of high voltage power lines. If we're talking residential interconnections, there won't be any issues burying those.

I would have thought they'd need to put sections of cooling in, if not maybe they have some form of oil or other liquid to help dissipate the heat? I thought one of the many reasons why the UK still does overhead between chunks of the grid is for cooling and easier maintenance.