I’ll stick to Cold War nuclear apocalypse ones. We’re talking about something which came very close to happening at times (either through accident or brinksmanship), and which would have been more devastating than the vast majority of hypothetical hordes of shambling undead.
Here’s a few of my favourites after the break. Depending on how your mind works you’ll either find these crushingly depressing, or you’ll suddenly find that nothing is quite as bad as you thought it was after watching them. One way to really impose scale on day to day struggles.