Vietnam: Cave of the Mountain River

The first river crossing of Son Doong at base camp with the upstream sump in the distance
The first river crossing of Son Doong at base camp with the upstream sump in the distance

Hang Son Doong — Cave of the Mountain River

TREVOR WAILES STC | This is a brief account of my part in the joint British-University of Hanoi expedition of 2010 to the truly remarkable Hang Son Doong (cave of the mountain river) in Vietnam.

The British-Hanoi University expeditions are usually every two years, but with the excitement of the Khe Son Doong discovery, a return trip in 2010 was planned.

In April 2009 an entrance 3 km downstream of Hang En was first entered. Our guide, Mr Khanh, had discovered it many years ago when foraging to supplement his family’s food supply; this was around the close of what the Vietnamese call the American War.

Our group of five was one of four groups that had left our village base at Son Track on a reconnaissance of the Khe Bang massif, a deeply incised limestone block with a general height of 850m but with enclosed river valleys often only 100m above sea level.

Our first night was at Hang En, a massive river cave 1.6 km in length, one of the best camp sites anywhere. It was explored and surveyed by the joint British-University of Hanoi expedition of 1994.

The river had been followed through the cave and down an enclosed valley to another entrance choked with logs (Log Jam Cave). Subsequent trips to this entrance had failed to overcome the blockage but the feeling was that it could be a significant piece of the jigsaw puzzle — that is, part of the Hang Khe Ry to Hang Toi/Phong Na system.

Mr Khanh led us to Log Jam Cave and we then climbed up the steep valley side to a relatively small entrance that dropped steeply down to where we could hear the distant thunder of running water.

In the subsequent days, we surveyed 5 km of extraordinarily large cave passage. Along the way were river crossings and two large sky light collapses, then mud and finally the great wall of Vietnam. Which looked like a 20m vertical climb. High above was a distant vague glimmer of daylight. From camp at Hang En to this point and back to camp with some surveying of about 1.3km took 14 hours.

Howard and Deb Limbert, who run the expeditions, were very busy planning some sponsorship and tried to sell the idea of a film or video documentary. National Geographic finally came to the party and pretty well financed the entire 2010 expedition.

I fortunately got the invitation to join them post filming and was possibly lucky in not having to endure six days of filming with a cast of thousands in the squalor of the cave and the surrounding jungle.

This episode is probably best forgotten but it was screened in January 2011 on the National Geographic Channel. The one plus that came from this enterprise was Sweeny’s (Gareth Sewell’s) bolt climb up almost 90 m of the great wall of Vietnam. This magnificent effort afforded another 700 m of passage to the entrance that had been suspected from below.

At one point, as he was drilling bolts into 8mm of flowstone with underlaying mud — sheets were plating off when loaded — a film producer asked if this was normal. Sweeny’s language crescendoed into a suggestion that they “F**k off and have a cup of tea.”

I joined the expedition at the end of this venture and had a fairly lacklustre month of jungle reconnaissance to distant caves up to four days’ walk away over some of the most inhospitable terrain I have ever crossed.

In the grip of El Niño

Water was a prime concern with Vietnam in the grip of an El Niño year, the porters and guides had to be very resourceful to get water from vines and banana trunks.

One such trip took our group to a massive vertical-sided collapse over an acre of jungle-floored shaft over 100 m deep. This turned out to be the Garden Of Edam, one of the two collapses into Hang Son Doong.

With hornbills flying noisily around at first light and bats silently at dusk, it was a memorable excursion. We did not descend Edam, but we were shown a shaft nearby which was surveyed to 100 m deep and some stunning photos were taken prusiking a shaft of sunlight.

This cave was on a side fault which should have connected with the major fault that created Hang En and Hang Son Doong. Maybe another trip to this area will confirm it.

Strangely, Howard wanted to have a personal photo trip through Hang Son Doong, despite having spent six days filming there. It was the end of the expedition and I thought many of the members who hadn’t been there would have been keen.

The guano-covered outflow entrance of Hang En
The guano-covered outflow entrance of Hang En

But it was Howard Limbert, Martin Colledge and I who prepared for a lightweight three-day trip through the newly discovered entrance to the Hang En end. Our main problem was that no one had ever been to the new entrance and the guides were sceptical about its existence.

We had a GPS fix on it — how hard could it be?

After an early start with Mr Khanh and another guide who knew the area, we were dropped off on the new Ho Chi Minh South Road at the start of the same track I had followed up to the Edam collapse.

As traffic on the road could be heard from the entrance, I thought it would be a short walk there, but the track went up the hill to about a third of the way to Edam before turning right along a cliff line and then started to descend through rough pinnacle karst back into the same valley. Watching our GPS, we could see ourselves nearing the entrance and then apparently being there! We were in fact 50 m above it. We had to hand-climb down a wall and then there it was — a huge entrance — 50 m by 50 m with cliffs below, left and right.

This was why we had spiralled around the ravines in the jungle. Contrary to what we thought, our guides were not lost. This jungle bashing had taken four hours and the road was 2.5 km away across the valley below.

The long slope down to a dry lake was rock-strewn with vegetation, mostly ferns, becoming sparser as we descended. The dry bed was pitted and compacted by the hoofprints of jungle deer; across the lake in the half-light rose a rimstone tier about 40 m high.

Atop this was a shallow gour pool with cave pearls (oolites) formed from the unseen drip line above our heads, with the jungle outside so dry that only an occasional drop spattered down.

In the pool were the calcified remains of an animal long deceased. Sweeny and his party, the first in here, had noted these remains and thought they were bear or jungle cattle. Mr Khanh thought they were almost certainly tiger and I can imagine a tiger lounging on this high point waiting for deer to come to drink in the lake below, but if it was in an El Niño year like this with no water and hence no deer, it would have starved to death.

The lower jaw was missing and the skull too ill-defined to make out any dental features. We left it disturbed only by the occasional splash of water.

The cave opened outward and upward as if it were a chamber rather than passage. The way on lay over rimstone and gour dams, all dry, the fine calcite crystal facets shining like ice crystals in our reflected light. Huge stalagmites tens of metres high broke the monotony of striding over the gours.

After 500 m from the entrance, accompanied by Mr Khanh and the guide, we reached the top of the Great Wall of Vietnam. Sweeny and Howard’s group had used climbing rope to scale the pitch (for the aid climb) and this had to be exchanged for a more suitable caving rope.

Mr Kanh would take the climbing rope back out to Son Track. There were three re-belays down the 90 m pitch, two of them on a 75º slope and the other vertical with a foot loop, so the whole pitch could be dropped with descender and cowstail. In the monsoon period the lower 15 m would be under water and was marked with a ‘tide’ mark around the lower passage.

We refreshed Khanh’s light from our supplies before he left, and we descended into the mud at the base of the pitch we had named Passchendaele. The ceiling of the passage at this point was about 200 m!

We regrouped, shared out gear and cut off 12 m of the climbing rope as a travelling line. Martin had carried in the replacement Bluewater rope and Howard and I shared the photographic gear. Last year this was the end of our surveyed cave and the trip out to Hang En had taken six hours.

Hang Son Doong Cave Map. Click for larger version.
Hang Son Doong Cave Map. Click for larger version.

Passchendaele was well named after the trenches of World War I. A mud trench led from a squalid mud tunnel under the Great Wall of Vietnam; the solid cave walls, 40 m apart, widened as we followed mud walls a metre wide lining the trench. Underfoot was knee-deep mud which sucked tiringly at our boots.

The vertical mud walls, 10 m high, continue for about 300 m until the rise in the passage floor, followed by another 100 m of slick mud slopes — not easy to climb with wet boots. Eventually, with the passage widening and rising, the mud subsides into gour dams, some with water. We washed off the mud and found a gour to fill water bladders — the first potable water available since we left Son Track. On our exploration of this area we had named this transition from gours and flowstone to mud The Sublime to the Ridiculous.

The water bladders added considerably to the weight we carried, but fortunately the campsite used by the film crew and extras was only 200 m away through some narrow rifts incised in the floor.

This gave way to more flowstone and compacted dirt, and finally a wall which had shed gypsum flakes to make a soft floor on which we could camp. The width of the passage here in semi-daylight was over 100 m.

The Cormorant extension was to our far left; this had been missed on our inward exploration a year earlier but we picked it up as we headed out. At 60 m wide and dropping down steeply to a static sump, it indicates just how vast are the dimensions of this passage.

The daylight was streaming in from the large entrance which framed the jungle-covered talus pile created by the collapse of the Garden Of Edam. The talus cone was maybe two acres and possibly 150 m high; the jungle green assaulted our senses and the scale of the collapse and sheer walls added to the mental confusion.

A week earlier, Martin and I had camped in the jungle 350 m above where we now were. One of our other groups was up there now; we could hear them and they could hear our calls, but meaningful conversation was lost in the distance and echoes.

One of the mega-stalagmite formations in this daylight zone is called the Dog’s Bollocks.

Looking out to The Garden of Edam and the Dogs Bollocks
Looking out to The Garden of Edam and the Dogs Bollocks

This first through trip was to give Howard the opportunity to take photos at a more leisurely artistic pace and he took many shots in the campsite area.

Models Martin and I headed out towards the talus cone across the soft gypsum. From where Howard was the track looked flat but the strain on our calves told us we were climbing quite steeply and on turning round, Howard appeared to be well below us. This was another optical illusion caused by the vastness of this feature. The film crew, porters, an entomologist and a geologist had spent four nights at this camp. A generator and 60 litres of fuel had been brought in to charge batteries; over 50 litres remained. Everything else had been removed.

Early the following morning, we packed and set off up the gypsum track into the Garden Of Edam. The vast jungle-covered talus cone was rough underfoot and the route lay around to the left rather than over its peak. The Alcove could have been mistaken for the way on, but previous exploration had shown it to be a rather extensive deep blind.

Further round the cone the Rat Run came into view, a steep descent with vegetation thinning as we dropped into the continuation of the cave.

We took photos beyond the dripline of a rather novel speleothem — a form of stromatolite, like a bundle of narrow pencils growing on rocks toward the light. Bacteria change the composition of calcite to form the rod-like structure. These forms can also be seen in Deer Cave and other entrances around the Mulu area of Sarawak.

In the Rat Run
In the Rat Run

We took more photos through the Rat Run, a straight, level passage totally floored and walled with broad gour dams, mostly dry. The smaller daylit shaft of Watch Out for Dinosaurs seen from Garden of Edam marks the termination of the 300 m Rat Run and also the end of easy caving.

This shaft appears to be a more recent feature, with less vegetation and a steep rubble slope marked with cairns, leading to a steeper descent over rotten gours and more loose talus into the dark zone.

A 3 m climb down to a step ledge overlooking a 45 m pitch above the thunderous sound of water had had a handline on it with maillons and bolt. Now only some tatty webbing and a bolt were left — just enough to descend with.

We passed down our packs and found that the ledge followed over some black voids and climbed back into the daylight zone. This is the other side of the Watch Out for Dinosaurs talus cone, a sheer wall of rubble treacherous to climb or descend.

It is fortunate that the handline bypass exists. The view presents itself as a valley of utter destruction; unstable blocks and rubble stretch out into the continuation of Son Doong.

The river we could hear so clearly in the bypass is nowhere to be seen. It is about 30 m to the base of the enclosed daylight valley, but our route heads steeply up the right-hand side over hand climbs and dubious rocking blocks. Howard says that some of the “track” is different; cairns have collapsed and recent falls have changed our route.

Guides and porters from the film crew have seen fit to take up caving to retrieve our necessary rigging handlines and gear, probably for profit. We still had two river crossings and a near-vertical entrance to negotiate. We took great care in crossing this destruction to the Level Playing Fields, an area of perched gours and rimstone on the far wall.

This had also been a transit camp and is sheltered out of the ever-present draught. The route onward is over loose talus piles, several of them over 40 m high. This was my fifth crossing, each time by a different route.

As the light fades into the dark zone, the passage widens to about 200 m. Below the scree slope the river can be heard and the route leads to huge stalagmite bosses, one of which, the Hand of Dog, soars into the distance above us. A wrong turn and you’re cliffbound, retrace your steps and you’re into more loose talus. 500 m on the talus gives way to sand. Last year an extensive pool — the Swimming Pool — had been full; now it was almost dried up. It marked the point at which we had to head for the river crossing.

The sand subsides at the river trench. In this area there are only two safe crossing points, and yes, the handline is missing.

A 4 m climb to the water’s edge and then to the far bank, 4 m across the fast-flowing river that has sculpted sharp blades beneath the surface. We used the 12 m cut-off rope and cross the river, Martin leading the way.

On our first trip into this area we were plagued with insects buzzing our lamps and faces, so it was aptly called the Fly Zone. Further back, towards the Hand of Dog, they had dispersed and the place was named No Fly Zone.

After a 4 m climb on to a flat shelf we could see the distant glimmer of daylight. The river in its trench often floods this shelf. There are no loose rocks, sand or pebbles; a short 2 m waterfall or cataract marks the beginning of the trench and is the source of the thunderous water sounds we heard at the entrance slope.

A short climb down into the pool that issues into the trench and a short wade/swim across to a sandy bank had us at base camp. A rope had been left on this crossing — vital when the water level is up.

Again, the rope and maillons were missing. 150 m upstream the river flows from a sump close to the surface sink of Log Jam Cave.

The base camp had housed a generator and fuel, along with two large boxes of huge screw-in photographic flash bulbs resembling light globes and some were missing. My imagination saw them being used in village huts with traumatic results.

Our outward journey was through, over and under a confusing pile of truck-sized boulders, past large stalactites and a pool of water mysteriously fed by a perched sump.

We were now in the daylight zone below the 50 m climb out. The route crosses from one side of the slope to the other, under overhangs to a greasy climb on flowstone.

Again Martin led and dropped the travelling line we had with us. We found a maillon, one of a dozen that were missing. We climbed with our own packs, Howard mindful of the worth of his camera and gear.

After a final climb in a stalactite-lined slot we stood at the exit of Hang Son Doong. Last year, on one of our survey trips in, a gale-blown cloud had issued from this entrance, cutting visibility to 10 m — very eerie, although 20 m down the climb it cleared to normal. The cave was creating its own weather.

Signs of the film crew’s porters’ camp were evident but it was still a 20-minute slog down a rough steep jungle track to the river and easier going. An hour later, wading up the river brought us to the huge back entrance to Hang En.

We had a quick trip through with river crossings and two 30 m mud-coated rubble piles liberally pasted with guano. We passed some hunting/fishing natives and we looked about surreptitiously but unsuccessfully for our missing rope before we finally came through to the guano-free east-facing entrance.

Mr Khanh handed us a cold beer. He and his offsider had carried in a slab of mixed beers for Martin and me and Coke for Howard. The fire was lit and there was rice and pork for our evening meal.


  • Without the cooperation of the people of Vietnam at both government and village level, these explorations would not be possible.
  • Satellite imagery shows that Son Doong and Hang En are on the same faultline. A lesser fault connects from the south-west and intersects at The Alcove at the Garden of Edam. It also shows there are larger faults in the Khe Bang massif to the east.
  • Both the Garden of Edam and Watch Out for Dinosaurs collapses are caused in part by the thinning of the deposits of the limestone beds.
  • What will happen to Son Doong and the jungle immediately surrounding it? The Mulu National Park has managed its resource very successfully. Rumours in Son Track involve cable cars through Son Doong or part of it. People will want to go there and resources like this will certainly be developed.
  • Son Doong is a major piece of the massif’s drainage pattern, but many mysteries remain. The river that sinks near the Hand of Dog and is accessed at Watch Out for Dinosaurs bypass was explored to a sump and it is assumed that in heavy flow it backs up to flood Passchendaele and in turn may flow through to Nuoc Nut. The main flow almost certainly goes through to Hang Thung, about 3 km distant.
  • Son Doong has been described as the biggest cave on the planet in dimensional volume — up to 200 m in height and width. Deer Cave in Sarawak has a massive entrance but it closes down. The Hang En outflow entrance is similarly large. The largest section of Son Doong is around the Hand of Dog scree slopes — 200 m wide and more than 100 m high.
  • I feel very lucky to have had a part in this and other explorations in the Khe Bang massif. If this is geriatric caving, it suits me well. Thanks, Howard and Deb.