For decades, 60 Minutes has explored some of the most remote and extraordinary places on Earth, places viewers have never been and likely will never go. But even by those standards, correspondent Scott Pelley's latest assignment stands apart. "This was completely unique," Pelley said. This week's journey takes viewers deep into the largest known cave passage in the world: Son Doong Cave in central Vietnam. Its caverns reach higher than 65 stories and stretch as wide as one and a half football