I don't know about you, but when I conceptualize this metaphor, what I picture is straight out of a cartoon -- like there's a man, he's walking down the sidewalk, without realizing it, he crosses over an open manhole, and he just plummets into the sewer below.
Ne znam za vas, ali kad ja zamišljam tu metaforu, vidim prizor kao iz crtića - čovjek šeta pločnikom i ni ne shvaćajući upada u šaht, stropošta se u kanalizaciju.