Broadway & 42nd Street, below ground seen from street, February 1903.

173rd to 177th Street under Broadway, April 1928.

A Circular tunnel with chains running across, February 1906.

175th to 183rd Street under Grand Concourse, Bronx, c 1928-1931.

A subway tunnel under construction at 4th Avenue & 15th Street, looking north from the street into the sunlit lower level, September 1901.

Looking east from Norfolk Street subway station, December 13, 1906.

The ground level of a Manhattan station, January 1931.

14th Street & 4th Avenue Pillars and tracks with stairs leading to the platform, October 1903.

Laborers look at the camera while working on a subway tunnel in Manhattan, c 1901-1931.

The subway on Delancey Street west, January 22, 1907.

New avenue open subway station east from Norfolk Street, December 13, 1906.

The station at 8th Avenue and 52nd Street, May 1927.

Workers man cranes at an interior structure in January, 1931.

Park Row and City Hall Park, from street level, November 1902.

By 1930, most of the 468 stations in existence today had already been built. From City Hall to modern-day Harlem, the underground liars - and the structures that surround them - would go on to form the nervous system of the city.Today, the system has 21 interconnected subway routes, the deepest of which travels 180ft below street level, at 191 Street in Manhattan. According to the Metropolitan Transit Authority, the web forms the world's seventh busiest subway system, behind Tokyo, Moscow, Beijing, Shanghai, Seoul and Guangzhou.These vintage photographs from theshow the New York City subway system under construction from 1901-1931.(via Daily Mail