11. Corner of West Houston Street and Varick Street, New York City, 1890.
12. A photograph of the now-damaged portion of Notre Dame taken between 1851 and 1870.
13. Opera Singer Mary Garden, 1908.
14. In honor of 4/20: the iconic Hollywood sign altered by Cal State Northridge student Danny Finegood as part of an art project (he got an A), 1 January 1976.
15. 20 years ago, the infamous Columbine High School massacre occurred on 20 April 1999.
Here, four young women headed to a library near Columbine High School where students and staff members were evacuated, after two gunmen opened fire inside a school in the suburb of Littleton, Colorado.