Conquering an Infinite Cave

There’s a jungle inside Vietnam’s mammoth cavern. A skyscraper could fit too. And the end is out of sight.

“Past the hand of dog, watch out for dinosaurs,” says a voice in the dark.

I recognize Jonathan Sims’s clipped, British military accent but have no idea what he’s talking about. My headlamp finds him, gray muttonchops curling out from beneath his battered helmet, sitting alone in the blackness along the wall of the cave.
“Carry on mate,” growls Sims. “Just resting a buggered ankle.”
The two of us have roped across the thundering, subterranean Rao Thuong River and climbed up through 20-foot blades of limestone to a bank of sand. I continue alone, following the beam of my headlamp along year-old footprints.
In the spring of 2009, Sims was a member of the first expedition to enter Hang Son Doong, or “mountain river cave,” in a remote part of central Vietnam. Hidden in rugged Phong Nha-Ke Bang National Park near the border with Laos, the cave is part of a network of 150 or so caves, many still not surveyed, in the Annamite Mountains. During the first expedition, the team explored two and a half miles of Hang Son Doong before a 200-foot wall of muddy calcite stopped them. They named it the Great Wall of Vietnam. Above it they could make out an open space and traces of light, but they had no idea what lay on the other side. A year later, they have returned—seven hard-core British cavers, a few scientists, and a crew of porters—to climb the wall, if they can, measure the passage, and push on, if possible, all the way to the end of the cave.
The trail disappears before me into a difficult pile of breakdown—building-size blocks of stone that have fallen from the ceiling and crashed onto the cave floor. I crane my head back, but the immensity of the cave douses my headlamp’s tiny light, as if I were staring up into a starless night sky. I’ve been told I’m inside a space large enough to park a 747, but I have no way to know; the darkness is like a sleeping bag pulled over my head.