SYSTEMATIC EXPLORATION

The Silent Pool was surveyed by renowned architect, caver and diver, Hendrik Viljoen, who mapped the caves in 1961 and did a brief exploratory dive to 60 m. He was so impressed, he even designed an underwater observatory for the site! By 1962 a new club had formed (BS-AC Branch 91; the Rhodesian Sub-Aqua and Speleological Research Group).  It consisted of divers who had specialist interests in exploring caves both above and below water.  It was time for systematic exploration of the cave system.  On this basis, we renegotiated access to the pool through the National Parks Department.

The framework
First, an accurate map was needed.  This started with a plane-table survey of the surface, showing the distribution of the sinkholes.  This was completed by first year geology students in 1961, as a mapping exercise.  This was built upon by the club by extending the map underground, mapping the caves above the water, the surface of the pool and plumbing its depths, resulting in a contour map of the underwater scree slopes and the depths of the shaft.  At last, we had a framework on which to hang details, as we discovered them. 

The overhangs were systematically mapped by dropping a shot-line at a known point and measuring off horizontally with a steel surveyors tape, using a depth gauge and a compass.  Measurements like this were only possible to a depth of 30 m; much below this, managing a team making accurate measurements was made difficult by short bottom times and the onset of nitrogen narcosis.  Beyond this, sketches had to suffice, which became more and more hazy as the depth increased.  The relatively small surface area of the Sleeping Pool belied the vast architecture of the water-filled cavern beneath.

The temperature of the water (22ºC) and the water chemistry were measured to 60 m by diver. Both chemistry and temperature are constant.  The water is slightly alkaline, typical of limestone, and is rich in phosphate (from bat guano).  The intense blue of the water is more related to its clarity than to any unusual chemistry.  A temperature probe was lowered to 85 m – still no temperature stratification.  The water temperature was almost constant the year round.  The biology of the pool ranged from single-celled protozoa, very small crustaceans, various insects, including dragonflies and damselflies, and their larvae, snails, frogs and their tadpoles, earthworms and a number of introduced goldfish, showing the consequences of a limited gene-pool. 

Much later, a game warden introduced a solitary baby crocodile into the pool.  We watched him grow, feeding off frogs and goldfish and we named him ‘George’.  He was no bother, and shied away from us, even on night dives.  Goldfish, which were never before seen below 20 m, were now seen at 45 m.  However, little crocodiles grow into big ones.  By the time he was 1.5 m long, he was less shy and used to bask in the sun, jaws agape, while we were diving.  His smile was a little too knowing, so we asked for him to be removed.

The scree slopes were combed for bones – some were found, those of baboons – no sign of human remains from the 1830s.  The water level varies over about 10 m and is dependant on rainfall and the extent of use of boreholes for the nearby hotel and irrigation.

Visibility varies with the seasons, reaching a horizontal maximum of about 100 m in August to November, just before the rains, when it deteriorates sharply to 30-40 m, due to mud being washed into the pool by summer thunderstorms.  Lighting also varies with the seasons.  Direct sunlight reaches the southern edge of the pool only at noon in winter, nourishing some waterweed and algae.  It was only in high summer and at noon that a beam of sunlight reaches the south end of the shaft and shines down it like a searchlight.  If this is coupled with a drought and resultant extraordinary visibility, the lighting effect is dramatic to say the least, converting the pool into the inside of a vast blue cathedral.

The nature of the bottom of the pool remained a mystery.  A scree slope of broken limestone blocks would indicate that the pool continued to still greater depths; a flat mud floor would indicate the pool did not go much deeper.  A mechanical grab was lowered to over 90 m.  It returned with a few sticks and stones locked in its jaws.  Any mud would have been lost during the long haul to the surface - inconclusive.

The Cave of the Bats
At the north end of the pool, another pool, which led into the Baboons Vestibule sinkhole, was accessed by scrambling over some boulders.  Here, the surface of the water was fouled by a thick mat of floating bat-guano and twigs that had fallen down the sinkhole from surface.  Beneath this, the water was dark but crystal-clear.  Surfacing under this layer did little for the hairstyle.  A quick exploration by diver showed the pool was about 13 m deep.  A small passage to one side, near the bottom of the pool, led into the top of a massive, but blind, vertical cleft.  Plotting this on the map indicated that it would probably exit into the roof of the Northern Overhang, the roof of which had been explored to about 65 m.  It remained to prove the connection.

On 13th January 1962, a team prepared to put two divers, John Inns and myself, through this shaft and out into the main pool.  We started in the Pool of the Bats, sweeping aside the odious surface crust.  We were roped together and roped to a surface tender so that, if the shaft did not connect, we could signal ‘up rope’ and return to the Pool of the Bats.  Jack Vauqulin followed us underwater to the entrance to ensure the rope did not snag.

Ian waves his torch at the start of the Bat Cave Dive

Quickly, we negotiated the short passage and entered the top of the shaft.  Our powerful dive-lights picked out the fretted black rock of the shaft walls that surrounded us.  There was no other light.  On shining my torch upwards, I saw a dense shower of brown particles and muddy water descending towards us, disturbed from the roof by our bubbles.  I was suddenly extremely glad of the thin white lifeline that connected us to the surface.  Without it, it would have been impossible to locate and exit the narrow passage at the top of the shaft, should our mission fail.  Despite this, failure seemed singularly unattractive, as a return in nil visibility would be tricky to say the least. 

We sank feet first rapidly past 30 m where the cleft widened to 8 m.  At about 45 m, we turned head down and it was with some relief that a rectangular slot about 3 m across of almost purple-coloured water appeared below us – the exit into the main pool.  We sank down to it and came out at 53 m, under the Northern Overhang into a blaze of purple-blue light.  The entrance to the overhang formed a massive black arch above us, complete with the fretted remnant of a huge, old stalactite.  Glancing back, we could see the murky cloud that had followed us, billowing and cascading out of the exit slot.  We untied the lifeline, signalled ‘up rope’ and began a leisurely ascent towards our friends Don Gay and Anne Smith, who were descending towards us from the Silent Pool.  There were handshakes all round and then the long, slow ascent and decompression. 

This dive indicated what could be done with proper preparation, training, risk minimisation and teamwork.  Later, our club put a permanent line in so other divers could make this exhilarating dive in comparative safety.

The Northern Overhang is about 15 m wide at its entrance and the roof cuts back and down in a series of inverted steps to just over 60 m where the sides narrow to about 5 m and it terminates in a vertical wall (Smith and Robertson, 11/3/62) at 70 m with no visible bottom.  Above is the entrance to the chimney linking to the Cave of the Bats.  The Southern Overhang has an arched roof.  It has been followed to 53 m, but appears to extend further.  To the west, it blends with the Western Overhang.  It is terminated in the east by the vertical joint plane of the eastern side of the shaft.

The Slot Cave and other caves
A side cave was found in the roof of the Western Overhang and it has proved to be one of my favourite dives.  Entry was at 21 m into a wide, black, arched entrance, which gives access to some narrow vertical slots running parallel to the north-south fracture that controls the cave system.  One of these slots plunges downwards.  Three divers at most, each hanging onto the leaders fins, would enter the slot, breathe out and sink down the slot in tandem, exiting under the roof of the Western Overhang at 43 m.  It was dubbed the Slot Cave.

A view S from 50 m under the N Overhang. Note the snorkeler, showing the clarity of the water, the guide line and the eroded remnant of an ancient stalactite

There are a few small cave entrances off the eastern side of the pool and fretted into the roof of the Northern Overhang.  These were explored but they narrowed and conditions became dangerously cramped and silty, so that a diver could proceed safely no further. 

The bottom of the shaft
As we became familiar with the shallower parts of the pool, our focus shifted towards exploring the deeper parts of the shaft and overhangs.  Vertical soundings of the shaft from surface indicated depths of 79 m at the south end, sloping down to 90 m at the north end where it passes under the Western Overhang.  A number of deep penetrations were achieved to 70 m or more.  On some, when the water was particularly clear, the bottom of the shaft could be dimly seen 20 m or more below; it had a ‘speckled’ appearance (Hanson and Robertson, 20/7/63, 65 m; Clark and Robertson, 4/2/68, 75 m). 

On one memorable dive on 27/12/67, four of us, Roly Nyman, Danny and Johnny van der Walt and I, reached the start of a scree slope at the bottom of the shaft in close to 100 m visibility, with the sun shining down the shaft, reaching a depth of 83 m followed by 27 minutes of decompression.  The east wall of the shaft fell sheer to the scree slope, which was scattered with the inevitable refuse from above – I remember an old torch, a tennis shoe and a waterlogged tree-trunk.  Nitrogen narcosis was extreme and my memories are hazy.  I looked at my depth gauge – the needle was just below the end of the scale.  I was unable to interpret my depth, and could only take a mental ‘picture’ of the gauge for later.  It was touch and back up the shot-line.  Later, I recalled that our group formed a good team to about 70-75 m but below this, the team came apart, each diver acting independently, each reaching a different point on the slope.  It was a dive I never ever wish to repeat and I think my buddies felt the same.  However, I don’t think such a dive could have been possible without a gradual ‘work-up’ to deep diving.

This slope was not the bottom.  The ultimate limit of the pool was a chimera that haunted and taunted us, defying our attempts, because we were up against human limits.  To press on any further would be completely foolhardy.   This was the situation until the winter of 1968, when I received a letter from the group of diving friends from South Africa who had accompanied me on the 83 m dive, with a most intriguing proposal.

THE FINAL PLUNGE TO THE BOTTOM

The proposal
It was a thick letter, in a brown envelope, and was from Roly Nyman, a long time diving friend, and Johnny van der Walt, a particularly capable diver and physiologist.  The letter began ‘Dear Ian, please read this letter at least twice before throwing it in the rubbish bin’.  It went on to propose a deep penetration dive to a maximum depth of 350 feet (107 m), using a helium-oxygen-nitrogen mixture (now called tri-mix but no one had even heard of tri-mix then). 

The letter gave a very detailed and full dive plan and justification of the gas consumptions and decompression procedures, including modification of standard decompression stops and bottom time to allow for 4000’ of altitude.   I was invited to be part of a team of four, with the two brothers, Johnny and Danny van der Walt, and Roly Nyman.  It was to be a co-operative venture between the Normalair Club of Johannesburg and our BS-AC club. I read the proposal once, again and even a third time.  I worked through the plan in detail with the helium tables in the US Navy Diving Manual and could find no fault with it.  They would bring a small recompression chamber and the tanks of ‘mix’.  We had to supply oxygen for decompression, a doctor and support divers.  It was a bold plan, there were risks, but it was well thought out.  Of course, I agreed and so did my club.  The date was set for between Christmas and New Year 1968 and planning began in earnest.  The objective was to find the flat mud floor, which would indicate the maximum depth of the pool.

This dive was a vast undertaking for an amateur club.  A mixture of 16 per cent oxygen, 36 per cent helium and 48 per cent nitrogen was devised, which struck the best balance between oxygen hypertension, narcosis and cost.  At 107 m (350 ft), the maximum target depth, the oxygen partial pressure would be quite safe for a short stay (by the standards in use in those days), and the nitrogen tension would be equivalent to a dive on air of less than 63 m, a depth at which the Deep Team were already quite at home.  At that time, the narcotic effect of oxygen was not recognised.

Decompression was to begin at 34 m and to continue, as laid down in the US Navy Helium Tables, after the necessary altitude corrections for the 4000 foot altitude of the pool were made.  Oxygen stops at 12 m and 9.8 m were planned to rapidly eliminate inert gases.  A clearheaded diver could accomplish useful work in a 10-minute bottom time and still endure the two hours of decompression in a standard wetsuit.

Just before Christmas, the Normalair Underwater Club divers arrived at Sinoia in force, complete with 35 SCUBA sets, a compressor, a 1000 kg recompression chamber, 22 divers, girl friends, and 9 precious bottles of helium-oxygen mixture.  Diving started in earnest.  It took almost a week to rig and test the mass of equipment including powerful, cable-fed, low-voltage, underwater lights, the shot-line, phone lines, the surface fed oxygen decompression supply lines and regulators attached to the shot-line.

The shot-line was particularly neat.  Its top part (to 34 m) was ladder-like, consisting of two ropes separated by white wooden bars, about a metre long, at each decompression stop and clearly marked with the depth and time of decompression.  At 34 m, the two ropes joined and a single rope continued to 60 m, terminating in a pulley.  The bottom part of the shot-line consisted of a heavy iron weight on the bottom passing up, through the pulley and down to a smaller iron weight.  Thus, the shot-line remained vertical and taut in the still waters of the pool, even if the weight slipped down the slope.  The oxygen line was run from the bank of cylinders and regulators in the Dark Caves to the top of the shot-line and down. 

Twelve metres from the surface, the black oxygen hose branched to six regulators, which were to supply the final decompression needs of the Deep Team.  Power cables to the underwater lights were run with the oxygen line down the shot-line, and continued to the bottom.  The underwater lights were lowered to the base of the shot-line at 90 m and switched on. 

A special large analogue depth-gauge had been built, cased in Perspex.  This was brought along on a 70 m dive to see if the lights were working.  They were, but the depth gauge imploded at 45 m with a loud ‘crump’, showering me in Perspex fragments.  A specially measured distance rope was devised and fitted on another 67 m dive. 

Each of the four-man team was individually subjected to the US Navy standard oxygen-tolerance test, consisting of 20 minutes at a simulated depth of 18 m on oxygen in the dry chamber, by the newly appointed Dive Controller, Jon Hemp.   All passed without an adverse symptom between them, indicating none of the team were over-sensitive to high oxygen partial pressures.  All were trained in recompression chamber operation.

Dive postponed
The day of the dive was set for the 28th December and the last preparations for the deep dive the next day were underway.  Suddenly the heavens opened and unleashed a tropical storm, which was violent even by African standards.  The hammering compressors filling the last tanks coughed into silence as ignitions flooded.  A torrent of muddy water swept down the cave and into the pool, where it spread like a bloodstain over the rain-spattered surface.  The visibility crashed to a record low of 8 m.  The dive was unanimously postponed for 24 hours.

Divers at the 37 m decompression stop

Concerned by the absence of our buddies, we returned to the shot-line and began the long rise to the surface. The light gradually increased.  At 45 m, we waved to the Deep Cover and began decompressing at 34 m where we were met by four cover divers.  Mike passed the phone to me.  My ‘Donald Duck’ attempts at talking on ‘mix’ caused much hilarity at Dive Control.

The rest of the decompression went ahead as planned (see below).  Boredom was the only problem during the 61-minute oxygen stop at 9.8 m where we were closely monitored by the cover divers for any sign of DCI or oxygen toxicity.  We switched from surface-fed oxygen to air for one minute in every ten to further reduce the likelihood of oxygen toxicity.  One hundred and twenty seven minutes after leaving the surface, the Deep Team surfaced very slowly, cold but elated.  We spent nearly an hour relaxing near the water before attempting the long, slow climb up the steps out of the sinkhole to the campsite.

The final assault
After the dive, it was decided to make a second attempt the next day with John and Danny's almost untouched tri-mix sets.  The dive was extremely successful.  John and Danny reached the rubble slope and followed it down, without delay.  At 104 m (340’), they reached a flat silt floor that extended as far as their torch beams could reach (6 m).  The depth of the silt exceeded arm's length. 

I was stationed at 45 m with Roly Nyman as Deep Cover.  The bubble screen pouring up the shaft from the divers below was massive.  When the two ascending divers came into sight, I saw they were buddy-breathing; Danny was almost out of gas.  I took over buddy-breathing with Danny when they reached us and continued to the first decompression stop at 34 m, where a spare SCUBA set was hanging.  The rest of the long decompression went without a hitch, entertainment being supplied in the form of assorted magazines, which became rather soggy but remained legible.

The TV Interview
This escapade was not without humour.  On the way back, we were invited to a live TV interview.  It was live because video-recorders were a thing of the future.  We were asked the usual questions but for one we were ready.  “What was it like breathing this synthetic gas mixture?”  Roly had brought a small tank of ‘mix’ and a regulator into the studio.  “It’s fine, please try it”.  The TV interviewer took a couple of breaths, opened his mouth and squeaked his impression.  The horrified look on his face was well worth the prank.

Conclusions
This diving programme proved the ability of a sufficiently organized club to carry out a deep mixed gas operation.  The gas mixture functioned perfectly and gas consumptions were very close to those calculated.  The flat silt floor at nearly 104 m might be the bottom of the cave, or a very large mud shelf.  Only an extensive survey would prove one or the other. These dives were completed without the aid of modern dive computers and BCDs and depended on exact dive planning and precise ballasting.

The completely calm, very clear, and comparatively warm water has allowed divers to penetrate deeper than in most ocean diving.  These excellent conditions simplify the diving environment.  Thorough training, a deep-seated culture of teamwork and good preparation all contributed to making it comparatively safe.  Nevertheless, the current PADI limits of 18 m for Open Water, 30 m for Advanced, 40 m for Deep and 50 m for Decompression divers seem, in hindsight, eminently sensible.  Repeated exposure to nitrogen narcosis, particularly over a short period, definitely allows the diver to manage it better and more safely, but only if the dive planning is detailed and the dive properly executed.

Looking back through my dive logs, various reports, old magazine articles and press clippings has been a fascinating but necessary journey to write this.  I still dream about a dive site that I know intimately, having some 360 dives there over more than four decades. While I was living there, no diver was bent, no diver was lost, despite the extreme depths and the hazards of cave diving.  When we left Rhodesia in 1975, my farewell dive was a Slot Cave dive. However this safety record has not continued. Chirorodziva has claimed the lives of two divers. With the advent of rebreathers, it would seem that the extent of the pool under the overhangs has not been fully explored.

Rhodesia, now Zimbabwe, is firmly in the grip of Robert Mugabe.  Since then, I have returned twice to Chirorodziva, once to take my youngest daughter for a very special 30 m dive at the north end of the pool, in December 1998.  On a later visit, in April 2000, three divers were keen to do a 60 m dive and I went along with them.  The team performed well to 50 m.  Although narked myself, it was interesting to observe how each member of the dive team became internally focused below 50 m; each oblivious to their buddies and staring fixedly at their dive computers.  This confirmed for me that 50 m is a sensible absolute limit for compressed air diving, particularly if there has been no preceding ‘work-up’ to deep diving.  Places where we entered the water in 1968-1975 are now at a depth of about 12 m, so the lovely Slot Cave dive is now probably impractical.

Dive organization
Deep Team:  Roly Nyman, Ian Robertson, John van der Walt and Danny van der Walt.

Dive Control Crew:  Jon Hemp (Dive Controller), Gill Pye (now Robertson) and Felicity Slabbert (Time Keepers), Tony Cobbett (Deep 'Phone), Lia van Domselaar (Surface 'Phone and also 2IC to Jon).

Deep Cover: Mike Dunwell.

Cover Divers:  First wave: Rowl Long, Pete Ziervogel, Pete Slabbert, Pete Kandy. Second Wave: Chas Stoneman (press representative), Ron Mitchell, Rod Shaw, Pete Jackson.

Snorkel Cover:  Frank Salt, Penny Salt, Berrit Long, Allen Cullen (Boat).

Shore Crew:  Ian Parsons, Madelaine Keet (Surface 'Phone), Bob Hammega, Louise Westwater (Doctor).

Chamber Crew:  Jon Hemp, Gill Hill.

Much of this was published in the October 1969 edition of TRITON (the then journal of the British Sub-Aqua Club) pp135-137. 'Helium dive into the Silent Pool' by Ian Robertson, Jon Hemp and Roly Nyman.

See also: -

Vallintine, R., 2003. The Club 1953-2003 - Fifty Years of BSAC Diving. Circle Books, UK. 280 pp. In particular pages 47, 79 and 118.

Condon, T. Beneath Southern Seas - published by Fin Diver Magazine. Article by John Hemp on pages 240-240 'Deep Dive Sinoia'