Travel through Vietnam's Son Doong, the world's largest cave passage

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Imagine, in the 21st century, discovering a marvel on par with Mount Everest or the Grand Canyon. It happened, in 2009, with the revelation of the largest cave passage in the world. It's in Vietnam and they call it Hang Son Doong, "Mountain River Cave." An intrepid British explorer, Peter MacNab, led the first team through this epic underworld of caverns the height of skyscrapers. MacNab is to caving what Armstrong is to the moon, the first explorer. Recently, we asked MacNab to show us this wonder of the world. But before we begin our trek, we really have to show you a preview of where we're going.

Simply glorious.

This was the moment Son Doong caught us in its grasp: sunbeams cascading 120 stories from a break in the ceiling, groundwater, above us, slipping through the light like rain — and rock reflecting what seemed like the only sound in the world. Not many have stood in this space that transcends time. It was a reward for our journey that began days before.

The only way to Son Doong is on foot—a trek of a day and a half. We had a party of 53 — moving in groups — mostly porters heaving camping and TV gear, plus experts in safety and climbing. There were 20 river crossings — water flowing through limestone, two of the essentials for building caves.

This is the slender center of Vietnam, the Truong Son Range between Laos and the South China Sea.

We were following the Vietnam War's Ho Chi Minh trail through a jungle where tigers are not unknown and leeches are plentiful.

Leading us were explorers Howard Limbert, whose work in Vietnam over 30 years discovered 500 caves. And Peter MacNab, whom Limbert sent to be the first in Son Doong.

Peter MacNab: I find it an adventure going exploring and not quite knowing what's around the corner and just sort of finding your way through. And things reveal themselves, like big chambers, big passages or tight narrow bits, beautiful formations.

Scott Pelley: And there aren't a lotta places on Earth that you can discover for the very first time.

Peter MacNab: No. You have to look pretty hard for them.

You have to look pretty hard for the entrance to Son Doong. You'd hardly notice but for the writing on the wall proclaiming the miracle of Ho Khanh. In 1990, Ho Khanh, a villager, discovered this entrance after he sheltered here from a storm. He told us:

Ho Khanh (English translation of Vietnamese): 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 cavers asked Young Ho Khanh to show them, but it took eight years, he'd lost it in the trackless jungle.

Ho Khanh (English translation of Vietnamese): In 2008, I finally found it. 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. Darkness would be nearly total, but we lit it so you can see. Peter MacNab was the first to do this. In 2009, he and four others on his team were dropping into darkness.

Peter MacNab: There's an obvious big black hole where you're heading towards, and you just sort of skirt around and look around and find, oh, this way'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.

Scott Pelley: You had no idea.

Peter MacNab: None whatsoever.

Scott Pelley: What was beyond the light on your helmet.

Peter MacNab: Yeah. 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 rappelled 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 Son Doong, the Rao Thuong river. Its waters are acidic so it's really good at dissolving limestone.

Scott Pelley: Well, this is a pretty good setting for an interview.

Darryl Granger: 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.

Darryl Granger: We found a nice package of sediment, further in the cave. And that dated to about 2.5 million years ago.

That's when the river first found a tiny crack in the limestone ridge.

Darryl Granger: 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 of 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 a wing. 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 cellphone, 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 "dolines."

The word has European roots; it means sinkhole or depression. And this doline 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 doline 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.

Scott Pelley: When my producer, Nicole Young, suggested this story, I turned her down.

Howard Limbert: Mmmhmm.

Scott Pelley: I said, "Nic, it's a hole in the ground."

Howard Limbert: Yeah.

Scott Pelley: What was I missing?

Howard Limbert: You're missin' best adventure that happens in the world.

Scott Pelley: Something no one's seen before.

Howard Limbert: That's the beauty of caves. If you're climbing a mountain, you can see where you're going, but in a cave, when you go in, you don't know what it's going to do.

Scott Pelley: Your father was a caver?

Peter MacNab: He was, yeah. So, he was a caver in Scotland.

Peter MacNab, the first in Son Doong, has been caving since he was a boy.

Scott Pelley: You grew up in a cave.

Peter MacNab: Not quite. But within-- a mile of it.

MacNab is a construction manager in New Zealand—big projects like hospitals --but you get the sense he does that to pay for this.

Scott Pelley: In all the caving that you've done, what is the closest call you've ever had?

Peter MacNab: 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 cavers call "a squeeze." MacNab couldn't back out. A partner found him and used a knife to rip away his coat to give him the spare half inch he needed.

Scott Pelley: Did he pull you out by your feet?

Peter MacNab: Pretty much. Yeah.

Scott Pelley: You are still exploring this region?

Peter MacNab: Yes, yes. We come back every two years, and we've barely scratched the surface of the caves in this area.

Scott Pelley: There may be another biggest cave in the world.

Peter MacNab: There could well be.

Truth is, MacNab's first expedition in 2009, never reached the end of Son Doong. Beyond this underground lake, he discovered a 30-story wall and ran out of time before he could scale it.

We climbed it on our trek and understood immediately why they call it "the Great Wall of Vietnam."

It's a 300-foot climb on slick rock with no foothold anywhere.

Scott Pelley: It's challenging enough until you realize, of course, you are doing it in the dark, and it's essentially raining. The ground water is coming from the roof.

So, with everything wet you find yourself slipping back while climbing up. But our team got us up and over, drenched and a little exhausted.

Scott Pelley: Well, we saved the best for last. We've made it all the way through, nearly, all the miles of the cave over three days and now we have just a little bit further to go. In fact, I can see the exit from here.

We could see it, that light up there, but we still had quite a climb to make. We learned, by the end of our trek, that Son Doong may be even larger than we know. Hundreds of feet below that lake behind us the water is draining somewhere. There could be more caverns beyond.

It's the work of millions of years likely to continue for millions more. Unimaginable — time — measured by a pendulum of light, illuminating the splendor of one of the greatest marvels on, or under, the Earth.

Produced by Nicole Young. Associate producer, Kristin Steve. Broadcast associate, Michelle Karim. Edited by Michael Mongulla.

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