Spider Martin's most well-known photograph, Two Minute Warning, shows marchers facing a line of state troopers in Selma moments before police beat the protestors on March 7, 1965. The day became known as Bloody Sunday. Spider Martin/Courtesy Tracy Martin hide caption

itoggle caption Spider Martin/Courtesy Tracy Martin

Yakutsk, Russia is the world's coldest city: average winter temperatures hit -30 degrees. It's also the largest city built on permafrost. Amos Chapple hide caption

itoggle caption Amos Chapple

The mother and daughter, displaced from their homes in Sudan, sit beneath a mosquito net while being treated for malnutrition at the Kalma camp in South Darfur. This is a 2005 photo. Lynsey Addario/Courtesy of Penguin Press hide caption

itoggle caption Lynsey Addario/Courtesy of Penguin Press

Gordon Parks wrote in his notes about the photo that "although the Negro woman serves as nurse-maid for the white woman's baby, the two would not be allowed to sit and eat a meal together in any Atlanta restaurant." Gino Domenico/AP hide caption

itoggle caption Gino Domenico/AP

Ephraim Benton, a former resident of Tompkins Houses in the Bedford-Stuyvesant section of Brooklyn, is now an actor. Benton started a community-based organization called Beyond Influencing Da Hood, which puts on health fairs, film festivals and various free community events in his old housing project. This photo was taken in front of his old building in Tompkins Houses. Courtesy of Shino Yanagawa hide caption

itoggle caption Courtesy of Shino Yanagawa

A boy lies on a mattress on the floor of Redemption Hospital, a holding center for Ebola patients in Monrovia, Liberia. Michel du Cille/The Washington Post/Getty Images hide caption

itoggle caption Michel du Cille/The Washington Post/Getty Images