A petroglyph emerges from the sands of the Sahara in Chad. Rocky skylines and dunes red with iron oxide make for a Martian landscape in northern Chad. The Sahara—the largest desert in the world—gets its name from the Arabic word sahra, or desert.

(Text adapted from and photograph shot on assignment for, but not published in, "Journey to the Heart of the Sahara," March 1999, National Geographic magazine)