Hang Son Doong - World's Largest Cave • Oxalis Adventure

The Son Doong Cave has a total length of nearly 9 km long and the volume is up to 38.5 million cubic metres, which makes this cave the largest natural cave on the planet. With this volume, Hang Son Doong is 5 times larger than Deer cave in Malaysia, which was said to be the largest natural cave ( before Hang Son Doong was discovered) with 9.5 million cubic metres of volume. However, what makes Son Doong special, recognized and well-known by the world is the unique hidden underground world inside the cave itself. It has many complex and giant stalactites (over 80m high), primaeval rain forests growing inside the cave, its own ecosystem, weather or a mysterious underground river that no explorer has yet discovered to the end of it. Many visitors admitted that the Hang Son Doong Cave is like another world, Ginger Zee from ABC News's - Good Morning America exclaimed that the sceneries in Son Doong were like those in the Avatar movie.

This amazing cave also houses an impressive ecosystem, inside the largest cave passage in the world. This huge and intricate cave system was created by the underground river, and filled with formations from the water that percolated down from a rainforest above. More of the highlights only found in this cave are:

• Hope and Vision Passage

  There is one of the largest sections of Son Doong; from the beginning of this passage, you can see the light from Doline 1, 1.5km away. Estimated by cave experts that a 40-storey skyscraper can easily fit in this passage and it is large enough for a Boeing 747 to fly through. Moreover, the world's tallest stalagmite, about 80m tall, is also located in this passage.

  
• Underground river

  There is an underground river inside Hang Son Doong Cave which is a combination of the water coming from En Cave and Khe Ry Cave (the longest underground river cave where the water comes from the Vietnam/Laos border area). From Son Doong entrance, you have to cross knee–deep rivers twice to reach the Hope and Vision Passage. There are emergency bridges in case of high water levels. The small waterfalls along the way create loud rumbles. Sometimes, the cave is filled with mist. This underground river disappears near Doline 1, about 4.5km away from the cave entrance. You can learn more about the underground river inside Hang Son Doong Cave here.

  
• Fossil passage

  Located next to the underground river at a depth of 40m below the campsite - Doline 1. This passage is imprinted with corals which have been fossilised hundreds of million years ago. This passage also has a 17 to 20 degrees Celsius underground lake in which visitors can swim and relax after a long hard trekking day. For safety reasons, life jackets and headlamps are required. Most visitors said that swimming in this cold underground lake is one of the best experiences that should not be missed in the Son Doong Expedition. You can read more about the Fossil passage here.

  
• Doline 1 - Watch out for Dinosaurs

  The collapsed ceiling of Son Doong formed a big opening to the outside, called Doline 1 (or skylight). Approximately 450 m high from the top to the bottom. This is also where the underground river disappears. On sunny days, there will be giant sunbeams coming in (from about 11 am to 1 pm), especially from January to March every year. Due to the solar orbit, the sunbeams shine deep inside the cave, illuminating the large passages. The beam from above the sky and the mist from the underground river create a majestic scene that can not be found elsewhere. Google Doodle has displayed this magnificent moment in Son Doong Cave on its search homepage. Many people said that it is like another planet, while others thought it was like the movie Avatar. Please learn more about Doline 1- Watch out for Dinosaurs here.

  
• Doline 2 - Garden of Edam

  Located about 1km away from Doline 1, this Doline has a primaeval forest growing 200 m deep inside the cave, below the limestone mountains. Coming out of Doline 1, you will see a small path leading to the forest. With a big opening on the top, this Doline allows a huge amount of sunlight to get into the cave, creating good conditions for many species of ferns, begonias, and even large trees, growing up along the path. You have to trek for 30 minutes to cross this forest to reach the second campsite, Doline 2. Please learn more about Doline 2 - Garden of Edam here.

  
• Cave pearls

  Cave pearls formed from drips of water from the ceiling that carries calcite carbonate rippling down to the gour pools on the floor of the cave. They come in different sizes and shapes, from as small as peas to as big as baseballs. The Son Doong cave pearls are naturally formed inside the gour pool and look gorgeous under the headlamps, especially on rainy days when water is running through them. These cave pearls are just ordinary calcium carbonate deposits that have no value, they will be discoloured and plain when taken outside or left under direct sunlight, as they will very rapidly dry out. Please learn more about Cave Pearls here.

  
• Passchendaele Passage

  There is a 600m lake below the Great wall of Vietnam. When the expedition team first approached the area, all of the water in the lake had drained out, leaving a hip-deep muddy passage. The members of the expedition team had to work really hard to go across this 600m muddy passage. It somehow reminded them of the movie about the battle of Passchendaele in World War I, the team decided to use the name Passchendaele to name this long and difficult passage. In many other months of the year or when it rains a lot, the water will fill up the passage to form a big beautiful jade-green lake. During the occasions when the lake is full of water, tourists will use rafts or boats to go through the Passchendaele. You can read more about the Passchendaele passage here.

  
• The Great Wall of Vietnam

  In April 2009, after nearly 10 days of exploration and surveying, the expedition team reached a giant calcite flowstone, blocking all ways out, they decided to return due to the lack of specialised equipment for climbing over this giant wall. Right after leaving Hang Son Doong Cave, based on measured parameters, the expedition team announced Son Doong as the largest natural cave in the world. No one knew what was behind that large stalagmite, at the time, therefore they called it The Great Wall of Vietnam to describe its impressiveness. Please learn more about the Great Wall of Vietnam here.