On 27 September 1889, workers put the finishing touches to the Tower Building. It was an 11-storey building that, thanks to its steel skeleton structure, is thought of as New York City's first skyscraper. The Tower Building is long gone – its plum spot on Broadway was taken in 1914 – but its erection marked the beginning of a construction spree that still has not stopped.
On the 300sq miles (777sq km) that comprise New York City sit 762 million tonnes (1.68 trillion pounds) of concrete, glass and steel, according to estimates by researchers at the United States Geological Survey (USGS). While that figure involves some generalisations about constriction materials, that prodigious tonnage does not include the fixtures, fittings and furniture inside those million-odd buildings. Nor does it include the transport infrastructure that connects them, nor the 8.5 million people who inhabit them.
All that weight is having an extraordinary effect on the land on which it is built. That ground, according to a study published in May, is sinking by 1-2mm (0.04-0.08in) per year, partly due to the pressure exerted on it by the city buildings above. And that is concerning experts – add the subsidence of the land to the rising of sea levels, and the relative sea level rise is 3-4mm (0.12-0.16in) per year. That may not sound like much, but over a few years it adds up to significant problems for a coastal city.