Vaulted sidewalk

Vaulted sidewalks are sidewalks that are not placed directly on the ground. Rather, there is an empty space below them where the ground level used to be. This happens in cities where the street level has been raised over time.
Chicago.
In Chicago, the city started raising street levels in 1855 as a response to the muddy conditions of the streets and because of epidemics of cholera. With the street level raised, there needed to be new, raised sidewalks to match them. In the case of vaulted sidewalks, which might be 5 feet or more over the original street level, a structure was built to hold a new sidewalk at the new street level, and an empty space was left between the original and the new sidewalks. This process led to a decision on the part of building owners: raise their buildings to the new street level, or relocate the main entrance to the second floor of the building to match the new street level. Many buildings chose the latter option, opting to use the vaulted area for storage. As recently as 2001 there were still over 2,000 vaulted sidewalks in Chicago
Today the old vaulted sidewalks are visible mostly during construction and cause increase costs of infrastructure maintenance. 
New York.
In New York, basements may extend beneath the sidewalk and drivers are warned "HOLLOW SIDEWALK, DO NOT PARK".