Robert Duncan poses with his wife, Karen, for New York photographer Iké Udé. Iké Udé/Courtesy of Robert and Karen Duncan hide caption

itoggle caption Iké Udé/Courtesy of Robert and Karen Duncan

Yes, those are people: A still image from a drone video filmed inside the world's largest cave shows cavers standing beneath one of two large skylights in Vietnam's Hang Son Doong. Ryan Deboodt /Vimeo hide caption

itoggle caption Ryan Deboodt /Vimeo

Planet: bottom of a glass containing half and half, water, food coloring. Moons: bottom of a glass containing coconut milk, water, food coloring. Stars: salt, cinnamon, baking powder, Tums. Navid Baraty hide caption

itoggle caption Navid Baraty

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