Phong Nha Cave

Late in 1899, on a quiet stretch of river in central Vietnam, a French priest sat in a dugout canoe. In his hand was a tiny oil lamp. People in the area had told him about a giant cave inside the limestone mountains. It ran so deep that no one had ever dared to go to its end. He decided to paddle in.

After more than 600 meters of struggling through the dark, he raised his lamp and froze. In front of him stood a stone stele carved with 97 strange characters. Beside it were an altar, fired bricks, pieces of pottery and old plates. Someone had been here a full thousand years before him.

The priest was Leopold Cadiere. And that cave was Phong Nha Cave.

Phong Nha is no ordinary cave. It lies deep inside the Trường Sơn mountains, at the core of Phong Nha Kẻ Bàng National Park. The whole cave runs for more than 7,700 meters. An underground river flows through it for nearly 14 kilometers. And that is just one cave. Across the whole national park, people have surveyed more than 220 kilometers of caves. That is 220 kilometers of tunnels under the mountains.

Visitors are allowed into only about the first 1,500 meters. The rest is still a secret.

Back to Cadiere. After he found the stele with its 97 characters, he wrote a letter about his discovery. He sent it to the director of the French School of the Far East, a French institute that studies the classical civilizations of Asia. Cadiere was so struck that he called Phong Nha the first cave of Indochina. He meant the number one cave of the whole region. Those were his own words, not an official title. Still, they show just how overwhelmed he was.

The stele itself waited much longer for answers. More than a century passed before Professor Arlo Griffiths established its date. The stele comes from the early 11th century, about 1,000 years ago. The ancient Cham used this cave as a place of worship. No one yet knows how they got in, or how they carried bricks and Buddha statues inside. And to this day, those 97 characters have not been fully deciphered.

That is one secret. Here is another.

In the early 1990s, a team of scientists from the British Cave Research Association came to Vietnam. They worked together with Vietnamese professors. The 1992 team had 12 British experts and 6 Vietnamese professors. That year, they measured Phong Nha Cave at 7,729 meters. Thirteen years later, another team surveyed Thiên Đường Cave nearby. In 2009, the explorers surveyed 20 new caves on a single expedition. One of them was Sơn Đoòng, the largest cave in the world and a familiar sight on television. It all began at Phong Nha.

In 1994, an international scientific conference on caves met in Hanoi. Experts from around the world placed Phong Nha among the most beautiful water caves, judged by seven criteria.

One, the longest water cave. Two, the highest and widest cave mouth. Three, the most beautiful underground lake. Four, the widest and most beautiful dry cave. Five, the most beautiful sandbanks and underground rock banks. Six, the longest and most beautiful underground river. Seven, the most magnificent array of stalactites.

Seven criteria for one cave. Seven.

The rock itself has an even longer history. The limestone mountains of Phong Nha Kẻ Bàng formed in the Paleozoic era, about 400 million years ago. Dinosaurs had not yet appeared. These are considered the oldest limestone mountains in Asia.

In 2003, UNESCO officially added Phong Nha to its list of World Natural Heritage sites. In 2015, it broadened the criteria for the listing. Then in 2025, it joined the area with Hin Nam No National Park in neighboring Laos. Together they became Vietnam's first natural heritage site that crosses a border. Two countries, one heritage site, and 217,000 hectares protected together.

Now, into the cave.

The trip begins at the Phong Nha boat landing, aboard a 12-seat motorboat. The boat travels up the Son River for about 30 minutes. Along both banks are villages, cornfields and limestone hills that rise straight up. In the dry season, the river is a lovely jade green. In the rainy season, it runs red with silt, because underground streams carry mud out of the mountains.

Then the mouth of the cave appears. It is 10 meters high and from 20 to 25 meters wide. It is a doorway nearly as tall as a three-story building, and wider than a road. Here the boatman turns off the engine, for two reasons. One is to protect the air inside the cave. The other is so that everyone can hear the water of the underground river.

The boatman takes up an oar. The boat glides softly inside. Lights set at various points shine on the stalactites. And here is the fun part. Long ago, people of the area came in with oil lamps. They named the formations after the shapes they saw.

One is Sư Tử, the Lion, because it looks like a lion's head with a mane. Another is Kỳ Lân, named after a creature of old tales. A third is Cung Đình, the Royal Court, for its rows of stone columns as straight as the pillars of a royal hall. On the ceiling is Mái Nhà Rồng, the Dragon Roof. There is Đức Phật, the Buddha, and there is Quan Âm, the bodhisattva of compassion. Those last two names are no accident. Deep in the cave lies the Bi Ký chamber, where the ancient Cham worshiped the Buddha a thousand years ago.

Good guides always make one thing clear. These are tourist names, given for natural shapes. They are not geological names. People made them up. Each person may see a different shape. That is the real fun of a cave.

After about 1,200 meters of rowing by hand, the boat pulls up at a bank of fine sand inside the cave. This sandbank is called Bãi Cát Tiên. Here visitors step ashore and walk into the dry cave called Cung Đình. Nearby is another dry cave, Tiên Sơn, 980 meters long. People of the area discovered it in 1935. Tiên Sơn does not connect with Phong Nha Cave. It has its own entrance on the mountainside, reached by climbing stairs.

These dry caves hold many formations with names like something from a fairy tale. Chuông Vàng, the Golden Bell. Tóc Cô Tiên, the Fairy's Hair. Buồng Cô Tiên, the Fairy's Chamber. Giường Vua, the King's Bed. Ruộng Bậc Thang, the Terraced Fields. Folk tales say that fairies often came down to bathe in this area. That is how the Fairy's Hair and the Fairy's Chamber got their names. This is folklore, not historical fact. It is still a lovely tale to hear.

In the 20th century, many visitors wrote and drew over the stele with 97 characters that Cadiere had found. Some of those marks are still there and cannot be removed. Each one is a piece of a thousand years of history, erased forever.

On the way out, the boat passes under small stalactites above the mouth of the cave. People often say these tooth-shaped stones, together with the wind blowing through the entrance, gave Phong Nha its name. Phong means wind. Nha means teeth. This is the popular account. Linguists have not reached an official conclusion. But stand at the mouth and hear the wind pass between those stone teeth, and you may find the explanation makes very good sense.

Three more details belong to this story.

The first is Road 20 Quyết Thắng. During the Vietnam War, this famous route linked the two sides of the Trường Sơn range. Its starting point, kilometer zero, is the Phong Nha boat landing. It is the very place where the boats leave for the cave.

The second clears up a common mistake. Many people think that Hang Chỉ Huy, the command cave of the Trường Sơn troops, lies inside this cave. It does not. The command cave sits at kilometer 12 on Road 20 Quyết Thắng. It is a separate cave, not part of Phong Nha Cave.

The third is about the map. Until 2025, Phong Nha belonged to Quảng Bình province. After the country reorganized its administrative units, Phong Nha became part of Quảng Trị province. It is the same cave. Only the province name on paper has changed.

This trip has crossed 400 million years of geology and a thousand years of Cham history. It has covered more than a century since Cadiere rowed in, and 20 minutes in the magical darkness of Phong Nha Cave. Phong Nha is a cave, and it is also a secret that each generation decodes only a little further.