Here's the tl;dr version: https://hackaday.com/2015/07/27/find-and-repair-a-230kv-800a...
Above ground is the way to go since it's cheaper to construct and much easier to service when things go wrong.
For example, if you'd simply stick a high voltage line into the ground, then there would be a voltage gradient between the ground voltage far away and (almost) the line voltage directly at the place where you stuck it; and since the earth isn't a very good conductor (compared to e.g. yourself), if you walk towards it, then the "step voltage" between your leg closest to the wire and the other leg can be sufficient to kill you.
That being unavoidable (vehicle on fire, etc.), don't walk away from the vehicle, jump, two feet at a time, and keep jumping that way, until you're a good distance away. This avoids you ending in a situation where you have that step voltage between the voltage rings.