Our planet is facing a mass extinction crisis. By the end of the century, we could lose up to half of all living species. But people are working hard to save endangered species and habitats, and a few scientists are even trying to bring lost species - like passenger pigeons and woolly mammoths - back to life. How To Clone a Mammoth - Beth Shapiro; Are You Ready for the Next Mass Extinction?; Recording America's Rarest Bird; From the Sabertooth to the Grizzly; Dangerous Idea: Tax Water Use; Writing the American West.