Short answers, pulled from the story.
The National Park Service began administering survey data gathered under previous legislation on the 9th of October 1960. U.S. Secretary of the Interior Fred A. Seaton declared 92 places eligible for designation on that specific date.
Sites must be where events of national historical significance occurred, where prominent persons lived or worked, or contain icons of ideals that shaped the nation. Outstanding examples of design or construction, places characterizing a way of life, and archeological sites able to yield information form the remaining criteria.
New York City holds more National Historic Landmarks than any other city in the United States with 116 NHLs according to available records. Pennsylvania, Massachusetts, and New York dominate these statistics alongside Virginia and California.
Fifteen sites reside in Puerto Rico, the Virgin Islands, and other U.S. commonwealths and territories while five locations appear in U.S.-associated states such as Micronesia. One landmark exists in Morocco known as the American Legation which represents the first National Historic Landmark on soil outside the United States.
Congress passed the Historic Sites Act in 1935 to change this fragmented approach regarding cultural heritage preservation. The legislation authorized the interior secretary to formally record historic properties and gave authority to designate properties having national historical significance.