Underground river in Son Doong

Rao Thuong upstream - the creation of the underground river of Son Doong

The underground river flowing into Son Doong originates from many small streams in the jungle along the km 33 to km 40 of the Ho Chi Minh Road west side. The stream is supplemented with more water from other sources Such as Cold Cave, Hang Ca Cave when flowing down the valley to Hang En. The stream is called Rao Thuong and flows through En Cave to where it is joined by the stream from Khe Ry Cave, increasing its volume. Hang Khe Ry is the longest river cave in Vietnam, with a length of almost 19 km and originates from the Vietnam - Laos border before flowing into Son Doong Cave. The collection of water sources from many different areas turns the Rao Thuong River into a big stream that has a large water flow and strong current on rainy days. And in the rainy season, around October every year, during big floods, the flood water can rise to 90 to 100m inside En and Son Doong Caves, and the water outside the valleys can rise more than 50m. The tree branches stuck high up on the walls and ceiling of the cave after major floods are proof of this. Usually, in limestone areas with large rivers flowing there will be caves formed by dissolving the limestone following joints and faults inside the bedrock. Currently, the underground river continues to flow inside Son Doong Cave.

The overview of Rao Thuong stream into Hang En Cave and Son Doong Cave.

The vanishing river

The underground river inside Son Doong Cave flows over cascades and waterfalls, some of which are 5m high and some are more than 10m high. The waterfalls create a loud rumble which can be heard even at the entrance. It was these sounds that made Mr Ho Khanh feel afraid when he heard a roaring sound when sheltering from the rainstorm at the entrance of the cave.

The underground river continues to flow inside the cave and the height of the cave in some places is 150-200m. Guests will follow paths in the cave, sometimes next to the river and sometimes 40-50m above the underground river. The Son Doong underground river continues to flow to Doline 1, where it descends 40m before disappearing from Son Doong Cave. The cave experts suspected that this underground river of Hang Son Doong connected with Hang Thung (a cave located 600m away).

In 2019, the expedition team of the world’s best cave diving expert (who participated in the rescue operation of the children's soccer team in Tham Luang - Chiang Rai, Thailand) led by Mr Howard carried out an expedition to find this connection. After nearly a week of surveying the expedition team discovered an underground passage at the depth of 90 meters. Unfortunately, due to the limitation of diving equipment for that great depth, the expedition had to be suspended. If the underground river passage is connected to Hang Thung, the volume of Son Doong will be added up to 1,6 million cubic meters. Further expeditions will be carried out in the near future.

At present, the underground river system from Son Doong Cave to Thung Cave is still a mystery for diving experts and the world.

The weather in the cave

Due to the difference in temperature inside and outside the cave, the air in the cave always circulates and forms cold winds blowing out from the inside of the cave entrance. It often carries with it a fine mist, formed from the steam from the river flowing through the cave.

The underground river in Son Doong Cave places a role of a thermostat for the whole ecosystem in the cave. The difference in temperature and water vapour from these underground rivers has created these cloud systems that make Son Doong magical in the eyes of explorers.

The Oxalis Experience.

FAQs

• is it safe to go to Son Doong during the rainy season?

  The rainy season in the Son Doong area usually occurs from September to November; during the peak of the season, the flooding level could go up to 90-100m inside the cave. With a deep understanding of the area; every year, Oxalis Adventure only runs tours from January to August. At this time, there would be little to no risk of heavy rainfall. In addition, the Operations staff are continually checking the weather forecasts.

• Is there any part inside Son Doong where we need to cross the river? And what would we do if the water rises unexpectedly?

  When guests enter Son Doong in good weather conditions, there will be two parts of the river to be crossed, both knee-deep. In case of heavy rain causing the water level to rise and making it impossible to cross the river, there is a steel bridge 10m above the water level. A safety harness will be provided when crossing the bridge.

• Would guests get trapped if the water rises so high that it reaches the camp?

  Oxalis specifically chooses to run tours during the time period when the risk of flooding is very low. Even during heavy rain, the water is estimated to rise up only about 20m. For Hang En campsite which is close to the water, there are a secondary safety camp 50m above the water level. The camp at Doline 1 of Son Doong is 40m above the underground river level, so it has proven to be very safe. The camp at Doline 2 is located beyond the underground river with no risk of flooding.