There is a shot of a janitor running a floor waxing machine in a highrise building. There is some loud noise & major rumbling. The power goes out & he is in total darkness. There is a door at the end of the hall with daylight showing underneath. He opens the door & sees that the rest of the building (& many others) have been completely obliterated. The camera pans back & shows him standing in the open doorway.
EDIT: The janitor opens the door at the beginning of this YouTube clip.