What lies beyond the reach of the naked eye, and how do these hidden forces shape our lives? This hour, TED speakers reveal forgotten cities, underwater canyons, tiny parasites, and a boiling river. In a special updated interview, space archaeologist Sarah Parcak explains how a global team of citizen scientists has helped her discover thousands of ancient hidden sites. Other guests from the original 2016 episode include computer scientist Abe Davis, ocean explorer Robert Ballard, science writer Ed Yong, and geoscientist Andrés Ruzo.