Son Doong Cave

Early in December 1990, a storm swept through the forest of Phong Nha. A man named Hồ Khanh hurried to find shelter at the foot of a cliff. He had come into the forest to look for agarwood, the precious fragrant wood of the mountains. Beneath that cliff was a small cave mouth.

He stepped closer, and the first thing he saw was mist. Wisps of mist drifted out of the opening, as if the mountain were breathing. Then he heard a deep rumble rising from the darkness. It was the sound of a river roaring inside the mountain. A cold wind blew out of the cave, and he decided to turn back. He did not go a single step farther.

He thought it was just a cave like the many others he had come across. So he made no effort to remember where it was. The storm passed, the agarwood hunter went on his way, and the breathing cave mouth sank quietly back among the limestone mountains.

Hồ Khanh was born in Phong Nha. The forests and mountains of home were where he made his living. Even so, for many long years, he never came across that cave mouth again.

Only in 2008, on another trip into the forest, did luck finally smile on him. The old cave mouth appeared before his eyes. This time, he would not lose it. He memorized the way by the shapes of the mountains and by the trees along the path.

In April 2009, he led a team of British and Vietnamese cave explorers through the forest to that very spot. The team was headed by the British explorer Howard Limbert. With him came geographers from the University of Science in Hanoi. Then the team stepped inside.

The river he had heard roaring years before turned out to be an underground river flowing through the cave. The team measured as they walked. The deeper they went, the harder the numbers were to believe. In places the ceiling rose about 200 meters high. In places the two walls stood about 150 meters apart.

Then, at the very end, a giant wall of rock rose straight up and blocked the way. The team had brought no climbing gear. They had to stop at the foot of the wall and turn back.

But what they had measured was enough to show something remarkable. The cave that an agarwood hunter had once stumbled on while sheltering from a storm was the largest cave in the world. That is how Guinness World Records entered Sơn Đoòng in its book of records. Later, once the whole cave had been surveyed, it was found to run for nearly 9 kilometers. It holds about 38.5 million cubic meters. One stretch is so big that a whole 40-story building would fit inside.

In March 2010, the team came back, this time with climbing gear. They climbed the rock wall that had stopped them the year before. Today it is called the Great Wall of Vietnam, and it stands about 90 meters high. Beyond the wall, the team walked about 400 more meters and reached the way out. The survey of the giant cave was complete.

The cave needed a name. Hồ Khanh and the experts chose the name Sơn Đoòng together. Sơn means mountain. Đoòng is the name of the valley where the Bru Vân Kiều people live. It is also where the Thượng River begins.

In the years that followed, Hồ Khanh began working as a porter for the groups going into the cave. Later he became the head of the Sơn Đoòng porters. The path he once walked alone now has groups of people following behind. Each group has at most 10 guests. Yet about 30 helpers go with them, carrying bags and supplies for the whole route.

The way to the cave passes through the village of Đoòng, where the Bru Vân Kiều people live deep in the forest. It goes past the mouth of Hang Én, the Swallow Cave. It crosses rivers and streams about 50 times. There are about 25 kilometers of forest and mountain trail. Then come 8 more kilometers inside the cave.

Inside stands a stalagmite with a funny name, Bàn Tay Chó, the Hand of Dog. Farther on, light suddenly pours down from high above. This is the first sinkhole, where the cave roof has collapsed and opened a patch of sky. People call this spot Vọng Khủng Long. Its English name is Watch Out for Dinosaurs. Around midday, bands of sunlight shine straight down to the cave floor. A thin mist often covers the sinkhole. The underground river flows this far and then disappears, about 4.5 kilometers from the cave mouth.

About 1 kilometer farther on is the second sinkhole. There, a primeval forest grows under the ground, 252 meters deep inside the cave. People named it Vườn Địa Đàng, which means the Garden of Eden. Thin clouds drift through the cave, and birds sing in the forest.

Near the underground river runs a passage called Hành lang Hóa thạch, the Fossil Passage. On its rock walls are fossils of coral, hundreds of millions of years old.

In small pools on the cave floor lie cave pearls. Water drips from high above, carrying calcite, which slowly builds up around grains of sand. Each grain grows into a round pearl. Some are as small as a pea, and some are as big as a baseball. They are not precious gems. Taken out into the sunlight, they fade and dry out. These pearls are beautiful only while they lie still in the darkness of the cave.

All of this lies in the core zone of Phong Nha Kẻ Bàng National Park, in Quảng Trị Province. UNESCO first listed the area as a World Natural Heritage site in 2003. It listed the area a second time in 2015. Each year, the cave also rests for 3 to 4 months, with no visitors, so that it can recover.

In the forest of Phong Nha, the cave mouth beneath the cliff still breathes out cold air. Inside, the river still roars in the darkness, just as it did on that stormy day. Only one thing has changed. The man who once searched for agarwood now knows what waits behind the mist: a forest under the ground, a rock wall 90 meters high, and pearls that keep their color only in the dark.