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Deep under the New York Bay near Brooklyn, covered in mildew, seaweed and other gunk, is what thousands of commuters would consider a hidden treasure: the start of a subway tunnel linking Staten Island to the rest of New York City.

“The idea of a subway to Staten Island really goes way back,” said Stan Fischler, a subway historian. “Way, way back.”

The plan, first proposed in 1890, was approved. Maps were drawn up. Groundbreaking ceremonies were held. Construction began.

What followed were dozens of missed opportunities that could have prevented the fastest-growing borough in recent decades from becoming the most isolated.