One of the spookier S-F short stories I've ever read, about a guy who's not paying attention while going down the escalators in a large department store and eventually finds himself in an infinitely descending series of sub-sub-sub-sub-basements with noplace else to go but further down.
Written by Thomas M. Disch, copyright 1964.
Reprinted in the 10th annual edition of The Year's Best S-F, edited by Judith Merril.