What does "air-bridged" mean? What is an "air-bridged harbor"?

Hello, I am wondering what "air-bridged" could possibly mean? I am wondering about it in the context of the Emma Lazarus poem, "The New Colossus." She refers to the New York Harbor as "the air-bridged harbor that twin cities frame." The twin cities that she refers to are New York and Brooklyn...Brooklyn was a separate city before 1898. However, I am not sure what significance the "air-bridged" phrase has?