It is one of the most memorable of all the many memorable images of Marilyn Monroe — standing over a sidewalk subway grate on Lexington Avenue as the whoosh from below sends her ecru halter dress billowing above her waist.

Ms. Monroe was the star of that scene, but the grate was undoubtedly a vital actor in a supporting role.

For nearly 110 years, since the advent of underground trains in New York City, the metal sidewalk grates have been about much more than mundane natural ventilation for miles of subterranean subway tunnels.

They have become urban artifacts, all 39,000 of them. They are the bane of women in high heels; a place for flicking cigarette butts, for expectorating chewing gum or for dropping valuables; a source of warmth to ward off a stiff winter’s wind; and a frightening opening to detour around.