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The Underworld of Mulu – edited from a split article originally written by Dave Brook and Tony Waltham and published in Caving International in October 1978 and January 1979.

In 1977-78 the Royal Geographical Society of Great Britain mounted a huge expedition to Mount Mulu in the north of Sarawak, Borneo. The mountain had been designated a National Park by the Malaysian Government as it is covered by virtually undisturbed rain forest from the plain near sea level to the summit at 2440 metres. The expedition, led by Robin Hanbury-Tenison assisted by Nigel Winser, was so organised as to provide support and facilities for workers to study all aspects of the ecology of the rain forest at various altitudes. The final objective was to draw up a management plan for the park.

Deer Cave entrance, Southern Hills. Photo © Jerry Wooldridge

Although the main massif is composed of sandstones and shales, a subsidiary range is massive limestone and culminates in Mount Api. The limestone mountain bristles with sharp pinnacles flanked by huge, sheer cliffs so that progress on the surface is difficult and hazardous. Rivers draining the Mulu Range have cut through the limestone in gorges up to 900 metres deep, and thus a group of separate elongate mountains have been created which tower above the surrounding terrain.
In 1961, lone geologist G.E. Wilford charted a few caves along the periphery of the mountains where access is possible via the rivers. No further exploration had taken place since that date and as the caves alone were a potential attraction for visitors the RGS invited a small group of cavers to pursue underground exploration, survey and research in the Gunung Mulu National Park. The group was Andy Eavis, Mike Farnworth, Phil Chapman, Dave Brook and Ben Lyon. Ben had to leave part way through the period, and we were then joined by Tony Waltham and Martin Laverty. During our three-month stay between March and June we had plenty to do as no searching for caves was necessary. Vast systems were just walked into with ease, and a total of 51 kilometres of passage had been surveyed by the time we left.
When the original caving team reached Borneo via the kind services of the RAF, the expedition base camp at Long Pala was reached by a 290 kilometre journey up the Baram, Tutoh and Melinau Rivers from the town of Miri. We were soon out in the field, or rather under it, on a visit to Deer Cave. This is undoubtedly one of the world’s speleological wonders, for our survey showed that the monstrous passage was 174 metres wide and 122 metres high in one section. It is nowhere less than 90 metres wide and high and it passes right through a mountain for a distance of one kilometre. A rock bridge occurs in the centre of the cave, and large avens soar above the general roof level. Because of its vast size, daylight penetrates virtually the whole cave, which simplifies navigation. From the south western entrance the stream may be followed along a wide sweep or the high road taken over a gigantic debris pile where ancient sediments are exposed. The col on top of the debris gives spectacular views of the first great section of passage. After 400 metres of simple progress the route is complicated by guano slopes, boulders and oxbows until daylight is seen streaming in from the far entrance. Two ways forward are possible: one involves a handline pitch down to the river, which flows into a wide sump and may be followed upstream and over boulders to a wide beach at the far entrance. The alternative route is up over guano-covered, very exposed ledges crawling with cave earwigs, followed by a scramble through pinnacles down to the beach.

Expedition base camp at Long Pala, now the site of the Royal Mulu Hotel. Photo © Tony Waltham

The main river from Deer Cave emerges from the parallel Deer Water Cave, where 880 metres of wet, but roomy passage were followed to an upstream sump about 90 metres distant from that in Deer Cave.  At the far entrance to Deer Cave a camp was established beneath the overhang as a base for further explorations. The enclosed valley upstream had been tentatively named the Garden of Eden, and it yielded several caves, but most didn’t go far.
One small stream sinking into the escarpment 180 metres north of Deer Cave, however, was the original way into Snake Cave, which proved to be a kilometre long. The stream soon ran into a choke where a climb up emerged in a larger fossil entrance chamber. A corkscrew route rejoined the stream and made for

Base camp at Long Pala. The cavers stayed in the small hut which was the first hut to be built on the site and nearer to flood level! Photo © Tony Waltham

easy going until a flowstone mass forced more climbing into a pleasant passage. Suddenly the way narrowed, and a snake reared up guarding the portal. It was all of two metres long, so exploration halted abruptly for the day. Next morning, it was still there waiting for passing bats, swifts or cavers! So a reinforced team equipped themselves with slip loops on long poles and ‘bagged’ the snake. The whole episode was like a far-fetched ‘Boys Own’ adventure. Beyond Snake Straights the cave ran true to its ‘Stranger than Fiction’ character as it enlarged to a fine cavern 45 metres wide and 60 metres long. The original stream sank into the floor further on, but at a boulder choke a larger torrent could be followed upstream to a cascade. Back in the cavern, high levels were explored to a deep canyon with an enticing rumble below, but it turned out to be the new upstream passage which soon choked. The Snake Cave water is presumed to feed a waterfall resurgence on the Melinau Paku River, and so it passes right through the retaining wall of the Garden of Eden.
Following the Snake Cave interlude, attention was turned to Green Cave, which had been found and partially explored earlier in the expedition by the Osmastons who were guided by Usang. The latter had a remarkable knack of finding mind-blowing caves.

An early river crossing. Melinau River. Photo © Tony Waltham

Subsequently, a team led by Tenison progressed a little further until stopped by a series of chasms. From the river entering Deer Cave the vast overhang above the top entrance of Green Cave can be seen, high under the rim of the Garden of Eden. The cave is approached by a very steep path which climbs 150 metres and ends abruptly at a parapet overlooking the vast entrance cavern, more than 60 metres wide and twice as high. Partway down the cavern a second shaft of daylight streams down a steep ramp up to the highest entrance beneath the overhang. Below the ramp the cavern plunges down before levelling out as a giant rift 90 metres high. A deep floor trench originates from an unexplored side passage. Progress is made by clambering over and around enormous stalagmites and by crossing delicate, fretted rock bridges. Below a long slope is a meandering stream which sinks under a mountain of flowstone. And now the atmosphere changes as silent progress is made across dark, dusty mudbanks in a passage where the roof is lost in gloom. The Osmaston cairn marks the limit of their exploration and the easiest way on was found to be at low level via a short climb. The high and low levels rejoin in a chamber strewn with fretted rock towers surrounding deep pits. Here, at the limit of the second expedition, the cavers took up the cudgel and braving yet another large snake, descended a series of ramps and shafts to a sump pool over 300 metres below the entrance. Across the shafts an airy route in the enormous rift was pursued until a vertical wall of infill 30 metres high seemed to end all hopes of progress. Most improbably, a series of small flakes on one wall provided a very exposed but easy way up and then along an arête with 30 metre drops on either side. The cave was rapidly taking on the atmosphere of Jules Verne’s Journey to the Centre of the Earth, for now a great pit 80 metres deep had to be crossed to reach the continuing passage. Once more the cave god smiled, however, and provided a road along a wide ledge for 45 metres across the walls of the pit. The tunnel beyond was 300 metres long to daylight at a series of windows in a near-vertical vegetated cliff. The 50 metre scramble down to the alluvial plain was as difficult as anything encountered within the cave and was a most satisfying way to end the journey under the mountain.
Two of the systems explored by Wilford contained large underground rivers, and these were pursued with enthusiasm because the water temperature of 27°C made exploration a joy in the wet, but a misery in the dry. The nearest river cave from the base camp was Cave of the Winds, which could be entered by boat, for it was right on the bank of the Melinau River. It proved to be 2.4 kilometres long. A 15 metre high and wide passage led to Wilford’s limit at a short swim into a confused area uniting eventually in the Inner River Passage. This terminated in chokes very close to the sink in the Melinau Paku Valley, which was the source of the river. Many high-level passages were explored, but the continuation of the old fossil cave was elusive. 

Heading up river, the longboat is hauled over the shallow rapids. Photo © Tony Waltham

More distant was the Terikan River Cave at the northern tip of Mount Benerat. Here Wilford explored several dry passages linking entrances on the edge of the flood plain with the underground river. Using lifejackets and ropes, our team of two continued to explore a mass of passages between the resurgence and the final upstream sump. Six and a half kilometres were surveyed, and only-lack of time prevented a search of the surface, which should find more entrances leading back into unknown parts of the river whose source is not known.
All caves investigated were not at low altitude, however, and one large river gathered on Mulu, ran into a 600 metre deep limestone gorge and sank in a blind valley in the heart of the Api massif. Such a challenge could not be resisted and we trekked for two days to reach the Hidden Valley.

Our earlier adventures in the incredible systems of Deer and Green Caves were hardly caving in the normal sense, being more akin to underground mountaineering. However, although we were not getting much strenuous caving practice thus far we were gradually getting used to the ridiculously hot and sticky conditions and felt it was high time we struck out into the mountains. One of the rivers draining Gunung Mulu entered a great blind cleft in the limestone mountain of Gunung Api. The unknown underground course of this lost river was the great enigma of the Mulu region and we couldn’t resist an early crack at it.

A nomadic Penan family in a tempory forest shelter. Photo © Tony Waltham

Dave Brook meets the Penan on a forest trail. Photo © Tony Waltham

Dave Brooke heading of into the forest | photo © Tony Waltham

A few weeks prior to our arrival in Sarawak, Robin Hanbury-Tenison and local guides had pioneered a route into this Hidden Valley. Robin walked back in with Andy and Dave to revisit some of the caves his group had found. It turned out to be quite a walk, first wading across the interminable swampy flood plain, then crossing numerous rivers in the steep foothills and finally surmounting a 1000 metre knife-edge ridge which guarded the southern approach to the valley. Because we and our helpers were well laden it took two days of sweaty slogging to reach our objective and get the camp ready for occupation.
The river sank into boulders in its bed and the dry channel could be followed to mud-choked flood sinks. All the accessible caves were short with no sign of the elusive river, but one satisfying touch was the exploration of a cave that Andy had noticed on the air photos. It was situated 50 metres above the normal river sink, and the entrance beneath a great overhang was a fossil sink of no mean size. The passage was 60 metres wide, although almost choked by sediments, and it was most confusing to explore, being less than one metre high in one region. Sadly, after only 600 metres the way ended in a draughting choke and the Hidden Valley river remained elusive.
Only the ‘easily’ accessible cave remained to be investigated so we scrambled up 100 metres through thick vegetation on the north wall of the valley to a most improbable hole in a draughting boulder pile. This was the entrance to Wonder Cave, which had been found in some mystic way by the remarkable Usang. Inside was a comfortable tube passage to a deep shaft with a chamber at the far side. A bold step overcame the obstacle, but another drop soon halted progress.
This was how exploration stood in Hidden Valley when Mike, Phil and Ben arrived with the main party. Robin’s group now left to complete the walk around Mt. Api, and the caving team carried on investigating other openings in the sheer walls of the valley. One pair revisited Wonder Cave and found a tortuous fissure for several hundred metres to yet another overhanging drop.
Ben had been taken ill the day after he arrived and was clearly not getting better so a note was sent back to base reporting his condition. By return of post the expedition doctor, David Giles, arrived in the valley and decided that Ben must be sent back to base at once. He couldn’t walk out, so a helipad had to be cleared. A site was chosen and we set to work with bush knives and hands. Finally, only one tree 1.5 metres in diameter and 30 metres high barred the way to success, but it succumbed to dogged chipping. There was doubt as to which way it would fall; if it came down across the pad two days of toiling would have been in vain. Our prayers were answered, however, and it fell true.
Radio communication was established from the 1000 metre ridge and the helicopter duly arrived to swoop in for Ben and Doc Giles, and drop off Robin with a chain-saw. The pilot must have been horrified at the tiny-clearing in the deep cleft of Hidden Valley, but he went through with it and returned later to retrieve Robin from a somewhat enlarged pad.
After this dramatic interlude food was very low so Andy decided to go back to base with the carriers and organize more supplies whilst we carried on over Easter. Wonder Cave was the only loose end left from the earlier explorations so the diminished team of Phil, Mike and Dave went in on Good Friday to clear up the survey. Little did they realize that one of the most memorable trips of their lives lay in store.
The first lead checked didn’t go so they dragged themselves through the tortuous fissure to the pitch. Phil shot down the drop, which was short but awkward, and was, soon back with the news that the way lowered to a glutinous mud crawl with a howling draught. We reluctantly glooped our way through the Roaring Forties and found that another sequence of airy climbs had to be made to follow the wind into the mountain. Suddenly the rift sidestepped and changed character completely. Until now the high, narrow passage had been descending steadily, but now at a depth of 90 metres a six metre diameter gypsum-encrusted tube shot up steeply to regain all the lost height. At the top we were halted at the base of a monstrous slab with a possible way up through precarious boulders. Being the lightest, Dave was volunteered to float up the Passover and report back. He found himself in a great, bouldery chamber sprinkled with small bushes of gypsum crystals. On climbing a mound of boulders for a better view, however, he realised that the ‘chamber’ was in fact the beginning of a great passage 50 metres high and wide.

The original Camp 5 in the Melinau Gorge. Photo © Tony Waltham

A euphoric exploration was halted by conscience at an eight metre lone stalagmite (The Watchman) in the centre of the tunnel. On hearing the astounding news, the others shot up the Passover to join in the march past The Watchman and down a flowstone glacier to a great pit ringed by frozen cataracts of calcite. Below was a miserable sump. Feeling cheated, we cast around for a continuation of the great tunnel. Mike made a fine lead up calcited fill to the roof level, and a way opened up past a great array of stalagmites to a 40 metre descent into the vastness of Calgary Cavern, whose floor was a carpet of calcite needles. We crunched our way along a path of least destruction beneath glittering formations; but 150 metres forward the way was solidly blocked by a wall of flowstone 50 metres wide and high — an incredible sight!
At the base of Calgary, however, Phil vanished into the uninviting but draughting Resurrection Crawl and found another huge cavern. Sadly, Mike was feeling ill so we slowly made our way out. Phil and Dave were back on Easter Sunday to survey and explore. The new cavern of the Elephants’ Graveyard was huge — 300 metres long, 90 metres wide and 110 metres high. A hike to the north end found a relatively small passage carrying our friend the draught, but on a later visit the cave was found to end in a chamber with inaccessible roof inlets. The last spectacular discovery in Wonder Cave was made by mistake when Mike and Andy were looking for the north passage. They climbed a 20 metre wall at the highest point to the Elephants’ Graveyard and found a boulder pile plastered with massive calcite fans guarding the entrance to the Moulin Rouge passage. The decorations found in the next 300 metres defy description; such was their variety and profusion. The way ended suddenly at a lake and flowstone blockage plastered with giant helictites.
In Wonder Cave we had an indication of the vast scale of cave development in Hidden Valley, but hard caving at 33°C for 14 hours was no joke and the Hidden Valley camp was hard to supply, so logistics dictated that we concentrate our efforts in the more accessible caves bordering the Melinau flood plain.
Back at base, Tony Waltham and Martin Laverty had arrived to strengthen our team, so we split into three groups. Mike and Andy had stayed on in Hidden Valley until a sick guide was well enough to travel; Tony and a still shaky Ben went to explore caves in the Melinau Gorge. Usang knew of a cave up-river so Phil. Martin and Dave went to have a quick look. We moored the boat where a river of clear green water joined the dirty   brown flow of the Melinau. The resurgence was a mass of boulders at the foot of the escarpment, yet only 30 metres up the hill a massive entrance was cunningly concealed behind a rampart of boulders. The incredulous trio could hardly believe their luck as they made their way into the darkness towards the roar of the Clearwater River and stood in awe on its bank. Upstream, a skylight 60 metres above illuminated a river canyon of staggering dimensions, but crossing the river to begin exploring proved quite a battle against the fierce current. As if in a dream we made our way up the river, crossing and re-crossing to use ledges resembling giant pavements. For 1.5 kilometres the passage was 30 to 40 metres high and wide with the river occupying much of the floor — a mindblowing passage by any standards!
At River Junction the waterway split and became deep so we cast around and found a way up through a screen of blocks into the great deserted passage of the Battleship, which rose to a draughting choke. By carefully threading our way upwards another mighty ‘ants in a sugar bowl’ sized cavern was found  but there was no obvious way forward apart from a hole in the roof, so the elated trio retraced their steps.
They were soon back to bivouac at the entrance and survey the cave as explored. From the last cavern a rather grotty and acrobatic tube suddenly popped out in a vast passage again. Success was short lived, however, for an abyss soon stopped us short. Sixty metres below was the River Junction, whilst across the frustrating void the continuation looked inviting. Something for the next expedition.
All the caving team assembled back at base, so a bivouac of several days was staged at Clearwater entrance so we could get some idea of the extent of this huge river cave, which we presumed was the outlet for the Hidden Valley water. All the wet passages from River Junction soon sumped, but a bypass series was entered low down in the Battleship. The second section of the river passage was lower, but still impressive for 500 metres until another sump the size of an Olympic pool barred the way.
We seemed to have been foiled at last, but no, an obscure rift was found to draught and it emerged in the floor of a great tube — Inflation Passage. The latter gave easy progress for a kilometre to a horrific climb into a tall east-west cavern (Detente). Here the way on was solidly choked by a 40 metre high wall of sediment — the end? No chance! Once more a loop-hole was found, for a passage heading back in the ‘wrong’ direction intersected a 40 metre square passage, aptly named Revival. The junction was guarded by a two metre long snake, alive and thriving on passing cave swiftlets. Its presence three kilometres from any entrance was quite a shock!
In this exciting fashion the second phase of exploration ended and we couldn’t wait to get back, but the weather caused a frustrating delay. At long last a team of three managed to force its way up the swollen river and noted with horror the flood mark more than two

Edge of the World, the end of the huge Revival passage in Clearwater Cave. Photo © Tony White

metres above normal. In Revival they surveyed on into the unknown and for two kilometres the monster tunnel maintained its dimensions. All the pits in the floor were bypassed with remarkable ease until we were stopped at the Edge of the World. A great gash cut across the passage and the chasm overhung on our side. We were back in the realm of Jules Verne yet again!
The next day was spent prospecting along the escarpment north of Clearwater. Many cave entrances were found and also a large sinking river, the Goldwater, which was later found to be a tributary to the Clearwater system and would have been a major cave in its own right in any other karst area.
Plans were now made for an underground bivouac in Clearwater Cave, since a steady walk to the end of the known cave and back took about seven hours. Carrying the bare minimum and surveying offshoots as we went, the full team met at the chosen bivouac near the previous limit of exploration. Partway down the Edge of the World a flowstone slope was encountered which led up into a continuation of Revival and a way out to daylight in 500 metres. Further exploration could obviously be made from a surface camp below the escarpment hereabouts, but was there enough cave left to justify it? We decided to work from the underground bivouac as originally planned and see how things went.
Below the Edge of the World were two kilometres of comfortable passageways and an easier exit to daylight just above the Goldwater River, but as usual the true way forward was found at the eleventh hour. High in a boulder chamber in Revival a small, muddy but draughting tube led to a pitch which was rigged by lashing slings to the bottom of the ladder ‘borrowed’ from the Edge of the World. We just reached the floor at a stretch and marched off along the 20 metre wide sandy tunnel of the Dune Series until halted by deep holes in the floor.
The cave showed no signs of ending so the Goldwater camp was established and a last attempt made to reach the end of the cave. Because our time was fast running out we were now caving in pairs in widely scattered areas, but this proved no handicap.
In Dune Series we found that the whole passage plunged down for 50 metres, and by good luck our bits of rope got us down by a devious route. At the bottom we wandered through a screen of boulders into the wide space of Great Wall Chamber, where a choice of ways had to be made. The more obvious West route soon required rope, but a return visit resulted in little progress as drop after drop gobbled up the line so laboriously carried in. We quit the vertical maze of the West route in favour of the boulder-strewn East route. As we progressed the passage expanded back to Revival dimensions, but after 700 metres a massive calcite choke blocked both high and low levels. Large stalagmites festooned this area and at last water was found for lamps, the whole of Dune Series thus far having been as dry as a desert.

Ben, Mike, stray botanist, DB, Andy on the base camp veranda at Long Pala. Photo © Tony Waltham

Close to the massive blockage another offshoot was discovered in the shape of a trench 40 metres high and ten metres wide tucked away in the corner of an oxbow. In true Mulu style it expanded to large dimensions, but at the end a short crawl led to a choke and a glimpse of daylight. Hoping to avoid the sweaty trip back through the cave, frantic digging began and two hours later a pair of somewhat shredded cavers emerged to rain and darkness. It should have been a simple walk back to camp, but in reality it was a seven hour epic for the floodplain hereabouts was a sea of limestone pinnacles into which all the surface streams soon sank. It was two very thankful cavers who found the camp at four in the morning.
In all, 24 kilometres were surveyed and 26km explored during our time in the Clearwater system. Many open leads remain and much more must await the next explorers. In particular, the river, which is lost beyond Inflation Passage, must rise 350 metres to the presumed sink in Hidden Valley. One thing is obvious — under Gunung Api lies one of the world’s largest cave systems whose exploration will remain a challenge well into the future.
Acknowledgements
None of the above exploration would have been possible without the Royal Geographical Society Mulu expedition and our thanks go to all who made that expedition possible; particularly those who organized the event both in Britain and Sarawak. The Malaysian authorities gave us much support and encouragement, particularly the National Parks and Wildlife Service. Especial thanks to our friends and guides of the Barawan and Punan peoples. Their assistance and knowledge was invaluable.