 This is The World. I'm Marco Werman. In recent months, we've been following the journey of National Geographic explorer Paul Salopek. He's been on a 24,000-mile trek across the globe on foot. It started back in 2013 in Africa's Great Rift Valley. Salopek is more than halfway through the trip that will end on the southern tip of South America in Tierra del Fuego. Of course, he's been photographing and recording the places and people he meets along the way.