So that's what those are! I've seen those atop hills along Highway 12 near Napa, California. Growing up in the area, I always wondered why there were what looke

So that's what those are! I've seen those atop hills along Highway 12 near Napa, California. Growing up in the area, I always wondered why there were what looked like blank billboards on those hills. Thank you, _Microft, for clearing up the mystery!