Vietnam's Son Doong cave is big enough for a skyscraper to fit inside | 60 Minut [5:01-10:05]

around and look around and find, and this way is pretty good, this way works. Quite often you get stopped, you can't get down here. You just basically feel your way through the cave by trial and error. You had no idea. None whatsoever. What was beyond the light on your helmet? Yep. No, we didn't at all. Every corner you went round was completely new, completely exciting, and it 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 Darryl 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 dated 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 hair, 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. It's 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 cave, this dough line is enormous. It's 450 feet above my head. In other words, about the height of a 45 story building. We stopped here with Howard Limbert, who's explored Vietnam since the 90s. When my producer, Nicole Young, suggested the story, I turned her down. I said, Nick, it's a hole in the ground. What was I missing? You're missing best adventure that happens in the world. Something no one's seen before. That's a bit of caves. If you climb into mountain, you can see where you go in. But in the cave, when you go in, you don't know what he's going to do. Your father was a cavever? He was, yeah. So he was a cavever in Scotland. Peter McNabb, the first in Sondong, has been caving since he was a boy. You grew up in a cave? Not quite, but within a mile of it. McNabb is a construction manager in New Zealand, big projects like hospitals. But you get the sense he does that to pay for this. In all the caving that you've done, what is the closest call you've ever had? I've been stuck. I've had rocks collapsed and I've been flooded in. He was stuck a few years ago when he went headfirst into a crevice that cabers call a squeeze.