Before the 70's, Battery Park City did not exist. The land that it now stands on did not exist and was, in fact, under the Hudson River. However, in the late 60's the harbor was becoming less useful because containerization was being demanded rather than break- bulk cargo. Simultaneously and conveniently, the World Trade Center was being built, and a place was needed to dump the earth from the building excavations, and so 92 acres were added to the bottom of the island of Manhattan and were called Battery Park City.
Initially, this area was planned to be a sort of escape from the urban environment of Manhattan, complete with a plaza and a lot of park space. However in 1979, the plans changed, and the area became an extension of the city, with more buildings. There is still however, a small park called Battery Park.