alt Hacker NewsOverhead conductors use air as the insulator. Underground cables use an insulating jacket. In the past it was really difficult to build cables with voltage ranges in the 10s of thousands of volts without additional complexity like a dielectric oil being pumped through the cable. I think modern dielectrics are significantly better though.
Modern cables with XLPE insulation can handle very high voltages without active oil cooling, here’s a 345/400kV rated underground cable assembly rated for 90C: https://assets.southwire.com/adaptivemedia/rendition?id=332f...