The Rebbe's Letter Concerning Manhattan's Bridges, Tunnels, and Boundaries, 15 Iyar 5714
The Rebbe's letter to Rabbi Hertze Eisenstadt treats Manhattan's bridges and tunnels as part of the actual enclosure question. A water boundary cannot be discussed while ignoring the crossings that pierce it.
An island with many openings
Manhattan is surrounded by water, but bridges, tunnels, piers, shore access, and built structures connect it to the wider city. Each crossing must be integrated into a halachic boundary.
The boundary must be traced opening by opening
Calling Manhattan an island describes its geography but does not complete the halachic analysis. The proposed wall must be followed around the entire perimeter, with each break identified and treated according to halacha.
That method applies to any city route. A map may give the appearance of enclosure while a bridge approach, railway, roadway, park entrance, or gap leaves the boundary open.
Why the details matter to the public
People who hear that a city is surrounded by water may naturally assume that the enclosure is secure. The Rebbe's questions show why the Rabbonim responsible for the eruv cannot rely on that impression.
The public heter is only as strong as the treatment of every crossing. Where those openings remain unresolved, the shoreline cannot be offered as a complete wall.