It's hard to imagine that anything literally hanging from utility poles across Manhattan could be considered "hidden," but throughout the borough, about 18 miles of translucent wire stretches around the skyline, and most people have likely never noticed. It's called an eruv (plural eruvin), and its existence is thanks to the Jewish Sabbath.
On the Sabbath, which is viewed as a day of rest, observant Jewish people aren't allowed to carry anything—books, groceries, even children—in public places (doing so is considered "work"). The eruv encircles much of Manhattan, acting as a symbolic boundary that turns the very public streets of the city into a private space, much like one's own home. This allows people to freely communicate and socialize on the Sabbath—and carry whatever they please—without having to worry about breaking Jewish law.