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The Phong Nha Cave System
 
The Phong Nha system starts about 40 kilometres south of Phong Nha Cave. Near to the Vietnam/Lao border a series of streams and rivers enters the limestone. The area was first accessed from the village of Ban Ban at kilometre 44 on the road 20. Heading east from the village a few small streams are noted. These all enter the limestone and find their way into Hang Khe Ry. The entrance to Hang Khe Ry is a large dry entrance. A large dry passage continues and soon leads to the main streamway. 
This cave forms a major part of the Phong Nha system. It is almost 19 kilometres long and eventually emerges in a small valley beyond Hang En.
 
 To the east of Hang Khe Ry, Hang Khe Thi is reached. This river joins Hang Khe Ry several kilometres into the system. The final sink to the east is Hang Khe Tien. This has only been explored for about 500m, but the water is believed to eventually connect with Hang En. This area should be checked out on the 2010 expedition.
 
Hang En is the next major cave in the system. A large river enters and flows through Hang En before joining with the water from Hang Khe Ry. Upstream of Hang En, a few short caves Hang Vu Ca Tau, Hang Khanh and Hang Hong form part of the system. Water from these caves joins the river leading to Hang En. Hang En has one of the largest sections of cave passage in the system. At one point the passage is 140m wide and at least 100m high. The cave is very spectacular. Exiting from Hang En, you enter an enclosed valley. The water from Hang En and Hang Khe Ry combine, and disappear underground in a mass of enormous boulders. Attempts to pass the boulder choke have so far been unsuccessful. There are a number of high level caves above Hang En and Hang Khe Ry. Hang Long, Hang Phong, Hang Doi, and Hang Ho Nui are all very well developed caves, but are not connected with the current drainage of the Phong Nha system. Often well decorated most of them end in calcite blockages.
 
In 2009 a very important discovery was made near the final choke of the Hang En River and the Khe Ry River. Above the final choke an entrance to a cave called Hang Son Doong (Mountain River Cave) was discovered and explored for 6.5k. This cave is huge with passage over 200m high and in places over 175m wide. This is the largest known passage in the world as yet discovered. The whole of the water from Hang En and Khe Ry combines to form Hang Son Doong. The river passage sumps but it is still heading away from the next known cave and thus still has large potential for more stunning river cave. The main phreatic passage is a huge tunnel and this is still continuing. At the end a 15m calcite wall stops the way on. Above is a passage around 100mx100m and daylight can be seen ahead around 500m in the distance.. This is the main lead for the 2010 expedition.
 
The next cave in the sequence is Hang Toong. This cave was explored in 1994. The water from Hang En and Khe Ry passes through Hang Son Doong before it is found in the 3 kilometre section of Hang Toong. The water emerges from Hang Toong, and continues to Hang Tra An. Until 2007, the caves of Hang Tron and Nightmare Shaft were unexplored. The exploration of these caves completes the link between Hang Toong and Hang Tra An.
Hang Tra An was first surveyed in 1992. It is about 600m long, ending in a sump.
In 2001, the team explored Hang Nuoc Nut. A dry entrance leads into a very well decorated cave and a large stream passage. 2.2k long the water emerges and flows above ground to join the Tra An river.
This large river flows on the surface for 4km until it enters the Phong Nha cave. The water disappears into a large jumble of loose rocks and tree trunks. The water enters in lots of places and we have been unable to find a way into the top end of Hang Phong Nha.
Above this area is the entrance to Hang 11. This small stream cave was finished in 2009 and must connect in some way to the Phong Nha system. Diving will be the only possible way into Phong Nha Cave from Hang 11.
 
At the bottom of the road 20, 3-4 kilometres before Phong Nha village, there are a number of small caves on the edge of the limestone. Hang Duc contains a small stream, and was explored for 1.3 kilometres. It ends in a large sump pool at the down stream end. This may also feed into the Phong Nha system.
Hang Phong Nha is 7.7 kilometres long. It has long sections of deep water passed by swimming, some sections of wading and walking along sand banks, and nearer to its exit some well decorated dry sections of cave. The first full exploration and survey of the cave was completed in 1992. In 2003, whilst working on the entrance to Phong Nha, the locals uncovered a dry entrance some 100m above the river entrance, Phong Nha Kho is a large dry well decorated section of cave 980m long which ends in a 10m pitch down to a lower level and a final calcite choke. This cave has now been opened up to tourism.
To the West of Hang Phong Nha lies the Hang Toi system, comprising Hang Toi, Hang E and Hang Hung Thoc. Hang Hung Thoc lies near kilometre 14 on road 20 and is 450m long. It is in an area which obviously floods in the rainy season. The entrance is very close to the end of Hang E. Water flows through Hang E 740m long, resurges and enters Hang Toi which is over 5 kilometres long, and is a very large impressive cave. It is believed that the Hang Toi system is formed by flood overflow from the Phong Nha system. When water levels are high, the choked upstream end of Phong Nha cannot take all the water, which is believed to overflow to the Hang Hung Thoc area where there are many places for water to sink.
The total length of the Phong Nha cave system is now over 62k.
 
Howard Limbert

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