Ever notice those 5-foot tall compressed nitrogen tanks on the streets around the city? With all of the crazy stuff going on in the world these days, and with New York a perennial target of people who want to do us harm, the sight of an oblong, shiny metal cannister will always produce concern among the citizenry. But, though ominous-looking, the are actually quite necessary.
The various utility companies place these tanks above ground and run lines down into the infrastructure for a number of reasons. The most common use is to pressurize pipes, lines, and wires. Below NYC is a maze of water, sewage, heat, cold, and other elements, and pressurizing these delivery systems helps to keep the lines safe from the elements.