https://www.reddit.com/r/fo4/comments/3shgfp/i_discovered_what_makes_the_game_so_performance/
What the fuck?
Edit: Reading the comments I guess it's not a big deal, but reminded me of that Crysis 3 tesselation thing where an ocean is being rendered underneath cities all the time.
I have a feeling the only reason he's seeing shadows rendered is because he clipped through and the game started rendering it. P sure what you're seeing there is the placeholder "skybox" geometry that you'd see at a distance from other quadrants, because due to the level of detail that's not really a notable strain.