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.
https://www.mentalfloss.com/article/91594/theres-wire-above-manhattan-youve-probably-never-noticed
