Inside the Tower; Unmanned; Wonder of the World | 60 Minutes Full Episodes [32:09-37:12]

I was collecting wood. I saw a sinkhole and I felt something strange. The strange feeling was wind blowing out of the ground. Cavers know that's the breath of a tremendous cavern. In 2000, the British cabers asked young Ho Khan to show them, but it took eight years he'd lost it in the trackless jungle. In 2008, I finally found it, he told us. In 2009, they started exploring. That exploration began here. We're just inside looking back toward the entrance above. The first obstacle is a spectacular 30-story wall that our climbing team showed us how to descend. And you step to the left side. Darkness would be nearly total, but we lit it so you can see. Peter McNabb was the first to do this. In 2009, he and four others on his team were dropping into darkness. There's an obvious big black hole where you're heading towards and you just sort of skirt around and look around and find this way. It's pretty good. This way works quite often you get stopped, can't get down here. You just basically feel your way through the cave by trial and error. You had no idea. None whatsoever. Almost beyond the light on your helmet. Yeah, no, we didn't at all. Every corner you went round was completely new, completely exciting and just kept getting better and better as you went into the cave. It was absolutely spectacular. Spectacular like the entrance we just repelled down. Look at the two men halfway down holding lights at the very top is the entrance and the last daylight we would see for a while. At the bottom of the climb, we met the architect of Sondong, the brow-tung river. Its waters are acidic, so it's really good at dissolving limestone. Well, this is a pretty good setting for an interview. Yeah. In camp, we spoke to Purdue University geologist Darrell Granger, who came here in 2010 to figure out when the river started its project. We found a nice package of sediment further in the cave and that they did to about two and a half million years ago. That's when the river first found a tiny crack in the limestone ridge. The width of a hare, maybe. Right, that's all it takes to make a cave. The water started flowing through it and dissolving it bigger and bigger and bigger. We still have water going through it today, so it's continuing to get bigger as we speak. Our exploration at the cave took three days and two nights. The length is 5.6 miles. Its 65 stories tall and the width of one and a half football fields. The great pyramid of Giza would fit easily. A 747 could fly through the biggest passage and not scrape away. Sometimes, the only way forward was the width of our shoulders. But we noticed, in the broadest caverns, you often lose the sense of even being underground. What reminds you is the isolation. No cell phone, no satellite. We were cut off from the world. Roughly halfway, there was a light ahead. There are two skylights where the roof collapsed. For us, a break from total darkness and a chance to show you the scale. Geologists call these holes dough lines. The word has European roots. It means sinkhole or depression. And this dough line formed because the roof over our heads, the limestone, is a little bit thinner here than it is in the rest of the cave. Then, as the cave grew wider and wider and wider over millions of years, it was unable to support the roof above. It all caved in right here. What's remarkable about it is that it allows light into this cave that would otherwise be utterly dark and it allowed the jungle to come inside the cave. Like everything else about the