In the early 1900s, Dr. T. Kenard Thomson proposed increasing NYC's property value by creating a land bridge between Manhattan and Brooklyn, building an island off the Jersey shore, and constructing a "New Manhattan" directly below the old one.
In 1916, Thomson, an engineer and urban planner employed by the City of New York, wrote a piece in Popular Science advocating a radical expansion of New York's land by reclaiming it from the sea. The plan would cost as much as the Panama Canal and included such flourishes as diverting the East River through Long Island (thus uniting Manhattan and Brooklyn), creating new land off the coast of New Jersey, and connecting Staten Island to Manhattan with underground subway tunnels. Said Thomson:
I propose to add, by a series of engineering projects, fifty square miles to Greater New York's area and port foothold. At the same time this will mean an addition of one hundred miles of new water-front. New York's City Hall would become the center of a really greater New York, having a radius of twenty-five miles, and within that circle there would be ample room for a population of twenty-five millions, the entire project to be carried out within a few years. Many have said 'It can't be done.' The majority of engineers, however, have acknowledged the possibility, and I have received hundreds of letters of encouragement.
Thomson estimated that construction would cost in the neighborhood of $50-100 million (1916 dollars, natch) per year (over an unspecified amount of time). When this plan failed to gain momentum, he scaled down his proposal. He would now affix a "New Manhattan" to the Battery. This new island would subsume Governor's Island and possibly the Statue of Liberty. The best feature was the "Four Mile Boulevard" that connected Manhattan and Staten Island; this boulevard included an airplane landing strip stacked on top of a road and railway in some crazed triple-decker sandwich of modernity.
You can read more about Thomson's design at Strange Maps.