Son Doong Cave

The mountains here are made of limestone. Water can slowly wear this rock away.

A river ran into a crack in the rock. Over a very long time, it carved a huge tunnel. Then parts of the roof fell in.

Below the forest, this part of Vietnam is one enormous block of limestone, says geomorphologist Darryl Granger in National Geographic. The whole region was pushed upward when India crashed into the Eurasian continent, 40 to 50 million years ago.

Son Doong itself formed two to five million years ago. River water flowing across the limestone sank down along a crack in the rock called a fault, and slowly scoured out a giant tunnel under the mountains. Where the rock was weak, the ceiling collapsed, leaving enormous holes called dolines that open to the sky.

In National Geographic's 2011 account, Purdue University geomorphologist Darryl Granger describes the region as one immense limestone block, uplifted when the Indian subcontinent collided with Eurasia 40 to 50 million years ago. Son Doong formed 2 to 5 million years ago, as river water flowing across the limestone burrowed down along a fault and scoured a tunnel beneath the mountains; where the limestone was weak, the ceiling collapsed into sinkholes, producing the cave's skylights.

UNESCO places the cave in a karst that has been evolving since the Palaeozoic, about 400 million years ago, with a diversity of cave types preserving abandoned river passages, changes in river routes and giant speleothems that were deposited and later dissolved again. The river still flows: tour operator Oxalis describes an underground river fed by water from Hang En and Khe Ry caves that disappears near the first doline, and National Geographic photographed a white streak marking how high water rises in the wet season.

Some animals live deep in the dark cave.

Scientists found white fish, white woodlice and white millipedes there. Under the skylights, the plants are like the ones in the forest above.

Two kinds of life share Son Doong. In the dark, biologist Anette Becher found woodlice, fish and millipedes that are completely white, which is common for animals that live in darkness. Under the skylights, botanist Dai Inh Vu found mostly the same mix of plants that grows in the forest above the cave.

The forests around the cave are rich too. UNESCO counts more than 2,700 kinds of plants and 800 kinds of animals with backbones in Phong Nha–Ke Bang, and notes that some small invertebrates and fish live in just one cave system and nowhere else.

National Geographic reports that German caver and biologist Anette Becher found white wood lice, fish and millipedes, a coloring the article calls common for creatures that live in the dark, and that Dai Inh Vu of the Vietnam Academy of Science and Technology found the plants under the skylights to be essentially the same assemblage as in the forest above.

UNESCO's statement for the property describes high endemism produced by the complexity of the limestone landscape, which created many ecological niches: endemic orchids and begonias above ground, and invertebrate and fish species restricted to single cave systems below. Phong Nha–Ke Bang alone recorded over 2,700 vascular plant and 800 vertebrate species at inscription, including 237 globally threatened species, and UNESCO cautions that actual diversity is probably much higher.

A man named Ho Khanh found the cave's door in the jungle.

Later, he showed it to a team of explorers. They climbed a giant wall inside. In 2010 they reached the other end!

Ho Khanh grew up near the park and knew the jungle well. The tour company Oxalis says that in 1990 he sheltered from a storm near a small cave entrance, felt a strong cold wind and heard a roaring river inside, but later forgot exactly where it was. Years later, he found it again.

In 2009, he guided a team of British cavers, organized by Howard and Deb Limbert and working with Hanoi University of Science, to the entrance. They explored about 4 kilometers before a muddy wall about 60 meters high stopped them. They named it the Great Wall of Vietnam. In 2010 the team returned, climbed the wall and found the exit.

Howard and Deb Limbert, from the Yorkshire Dales, first caved in Vietnam in 1990 with the University of Science in Hanoi and, by National Geographic's account, helped Vietnam establish Phong Nha–Ke Bang National Park. Ho Khanh, a local man who knew the forest from years of working in it, had found Son Doong's entrance but lost it; Oxalis says he first saw it in 1990 and relocated it in 2008, while National Geographic says he found it as a boy.

The spring 2009 expedition explored about 2.5 miles (4 km) before a 200-foot (61 m) wall of muddy calcite, the 'Great Wall of Vietnam', blocked the way. In 2010 Gareth Sewell, belayed by Howard Clarke, spent about 20 hours over two days drilling and climbing the wall, and the team reached the far entrance. Guinness World Records listed Son Doong as the world's largest natural cave in 2013, Oxalis reports, and Oxalis began running tours that year.

Only a few grown-ups can go inside Son Doong. It is a very long and hard trip.

Families can visit other caves in the park. Always go with a guide and wear a helmet.

Son Doong is not a cave you can simply walk into. Tours are run by the company Oxalis Adventure, and the expedition takes several days of hiking, river crossings and camping inside the cave. On the first day alone, guests trek about 14 kilometers and cross rivers around 30 times. A team of about 30 guides, safety experts, forest rangers, cooks and porters goes with every group.

Everyone must attend a safety briefing and wear a helmet. Families can enjoy easier caves in the park, such as Phong Nha Cave, which visitors reach by boat. Wherever you go, go with a guide and never enter a cave alone.

Access is tightly controlled. Oxalis Adventure, which opened Son Doong to visitors in 2013, runs a 6-day, 5-night package that includes a mandatory safety briefing and signed release, equipment checks, a 14 km first-day trek with about 30 knee-deep river crossings and steep slopes, and two nights camping inside the cave, with a team of about 30 guides, safety experts, forest rangers, chefs and porters per expedition. National Geographic described slick boulder slopes in the cave where a misstep could mean a fall of more than 100 feet.

In the monsoon, underground rivers in the area swell and flood passages, National Geographic reports. UNESCO warns that tourism pressure must remain within the ecological carrying capacity of the karst, so the strict limits also protect the cave's fragile formations and life. Visitors to any cave help by staying on the route, never touching formations, and carrying out all their litter.