Let's say that a mega-asteroid is heading towards Earth. I want it to destroy essentially the entire population except maybe a few million people around the world. Now, I know that for an asteroid of this size, we would like have a couple of years of warning at the very least, but let's say that in the chaos and panic, many people die, and not everyone gets to escape.
Those that do, what would be the best way to survive the explosion? Underground vaults deep beneath the soil? Underwater habitats maybe? How long would they have to stay there until it was time to come out, given that there were likely be a cooling period as the debris blocks out the sun?