Chris Stephens, 28, who has been battling depression all of his life, plays with his dogs at home in Concord, Calif., on Friday. After a dose of ketamine, Stephens says, "I actually wanted to do things. I wanted to live life." Lianne Milton for NPR hide caption

itoggle caption Lianne Milton for NPR

Nonnative pythons, like this one, are invading the Florida Everglades. As a top predator, the snakes have crippled the populations of rabbits, raccoons and other animals. Joe Raedle/Getty Images hide caption

itoggle caption Joe Raedle/Getty Images

Ketamine has been used as an anesthetic for decades. It's also a widely popular but illegal club drug known as "Special K." When administered in low doses, patients report a rapid reduction in depression symptoms. Huw Golledge/flickr hide caption

itoggle caption Huw Golledge/flickr

The Delboeuf illusion makes one dot appear larger than the other. But they're the same size. Your brain is misled by comparing the dots to the surrounding circles. Washiucho/Wikimedia Commons hide caption

itoggle caption Washiucho/Wikimedia Commons

Without a centralized national repository for nuclear waste, the radioactive material is currently being kept at various sites across the country. Above, large concrete canisters, each holding 14 55-gallon drums of waste, are loaded on a truck in 2005 in Richland, Wash., where they were later shipped to a facility in New Mexico. Jeff T. Green/Getty Images hide caption

itoggle caption Jeff T. Green/Getty Images