As mentioned with the other guy, 0 inches is the requirement to the top of the cable or raceway used. So at minimum, you’re allowed to be perfectly flush with the ground. Obviously you can and likely would go a little lower, although I don’t have any experience with trenching myself.
I would totally say “I saw insert thing on the way to work today,” and would have assumed that would be the natural way to say it, but now I’m questioning that