Manhattan's Water Boundaries, Bridges, and Tunnels
Manhattan's water boundaries gave it possible enclosure advantages that many other large cities do not have. The crossings remain the central difficulty.
The shoreline as a possible wall
Manhattan is surrounded by rivers, but the waterline is not uniform. Seawalls, steep banks, shallow edges, piers, parks, beaches, and points of public access can have different halachic status.
A valid boundary must follow the physical edge section by section and show how each part joins the next. Blue water around the island on a map does not create one continuous wall.
Bridges and tunnels pierce the boundary
Every bridge and tunnel creates a public route through or over the claimed enclosure. Vehicle lanes, ramps, approaches, pedestrian paths, service areas, and open ends all require treatment.
Manhattan's crossings are especially significant because they connect the island to the continuous streets and population of the wider city. They cannot be treated as small gaps in an otherwise closed shoreline.
The Rebbe's questions about the physical route
In his letter to Rabbi Hertze Eisenstadt, the Rebbe called attention to the bridges, tunnels, and openings that had to be addressed. His approach was concrete: the enclosure had to be tested against the actual city infrastructure.
Those questions remained part of the Manhattan controversy even though the island possessed stronger natural boundaries than many other cities.