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UNBEKNOWNST TO MOST there are transparent wires hang over 200 cities in North America.

And they have nothing to do with electricity.

Why are they there? They mark a religious boundary.

And every week, there is a ‘secret operation’ to check and repair the wire in time for the weekend.

Howard Shapiro uses poles to build an eruv on the boardwalk of Venice beach in Los Angeles AP / Press Association Images AP / Press Association Images / Press Association Images

Business Insider took a closer look at exactly what they are.

“The idea of creating holy space is really an idea that people generally appreciate today,” says Rabbi Adam Mintz, the co-president of the Manhattan Eruv.

“One of the prohibited activities on the Sabbath is something called ‘carrying’ (editor’s note: yes, this simply means carrying any item in your hand). The rabbis realise the difficulty in enjoying the Sabbath if you weren’t allowed carry outside of the house.

“Therefore they created a construct in which the area in which the Jews lived was enclosed – first by a wall or a fence, then eventually by a symbolic wall or fence which was created by setting up two poles and a string on top.”

waltarrrrr waltarrrrr

The eruv was born.

And that imaginary wall served to religiously enclose the neighbourhood and to allow Jews to carry within that neighbourhood.

In 1970, there were not more than 10 eruvs in North America. Now there are at least 200 of them.

With maintenance costs of $100,000 per year, the Manhattan Eruv is the most expensive in the world. It encloses almost the entire borough. It has never been down – surviving even the Thanksgiving parade floats and Hurricane Sandy.

The Lower East Side eruv cuboctahedron cuboctahedron

This is not an old tradition. The eruv comes into play for Orthodox Jews every single week.

A rabbi checks the entire enclosure on Thursdays, testing that the strings and the poles are exactly where they should be. He identifies repair needs, telling the maintenance company of any problems. They set to work on Friday morning to ensure everything is ready for the Sabbath.

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