The 1973 Mount Gambier cave diving accident was a scuba diving incident on 28 May 1973 at a flooded sinkhole known as "The Shaft" near Mount Gambier in South Australia. The incident claimed the lives of four recreational scuba divers: siblings Stephen L. and Christine M. Millott, Gordon G. Roberts, and John H. Bockerman. The four divers explored beyond their own planned limits, without the use of a guideline, and subsequently became lost, eventually exhausting their breathing air and drowning, with their bodies all recovered over the next year. To date, they are the only known fatalities at the site. Four other divers from the same group survived.
The incident was influential in the restriction of access to cave diving venues in Australia, the formation of the Cave Divers Association of Australia later that year, and the development of the South Australian Police Underwater Recovery Squad.
The Shaft
The entrance to the sinkhole known as "The Shaft" was discovered in 1938 in a field known as Thompson's Paddock, on a farm near Allendale East, a few kilometres south of Mount Gambier, when one of a team of horses stumbled over a small hole about a across, which proved to be a solution shaft opening into a large sinkhole. The hole was later opened up to about diameter for exploration access. In the mid-1960s a local diver climbed down the rain-enlarged opening into the cave’s wide lake chamber and made the first descent to a depth of about .
The cave is one of several deep solution features in porous limestone in this part of south Australia. The main chamber is about long and wide. Water level is about below ground level, and the hole is considerably wider at this point. A rock pile directly under the surface opening rises to a minimum depth of about . A tunnel extends to the northwest to a depth of about , and another to the east reaches a maximum depth of . Because of the size of the hole and the height of the drop, divers and equipment are lowered into the chamber separately using a hoist system.
The name is claimed to be derived from the bright shaft of sunlight which penetrates through the entrance to the depths on a sunny day.
The first reasonably accurate map of the cavern was made in 1984 after the newly formed CDAA Research Group made mapping and photographic dives showing the chamber to be roughly oval in shape. There was another exploration in June 1992, but the deepest sections were surveyed and mapped in 2002/03. The eastern tunnel is obstructed by restrictions at about and continues downwards to before becoming horizontal, and further access is blocked by a rock pile. Despite knowing their maximum depths, however, the total lengths of the northwest and southwest tunnels are unknown, and they may extend as far as the southern shores as part of the intricate freshwater-drainage system flowing through the mountains in the area.
The Shaft is on private land and is restricted to divers accredited with the CDAA.
Background
On 26 May 1973, a group of nine divers arrived at Mount Gambier in South Australia with the intention of cave diving at "The Shaft," a wide sinkhole opening leading to a large underwater cavern, famous among divers for its clear water and interesting internal structure. John H. Bockerman, Peter S. Burr, Christine M. Millott, Glen Millott, Stephen L. Millott, Larry Reynolds, Gordon G. Roberts, and Robert J. Smith were the eight divers intending to venture into the cave; a ninth group member, Joan Harper, had decided she would not be diving, and would instead remain beside the sinkhole, preparing hot soup and assisting the group in other ways.
The day prior to the accident, the group signed the guestbook at the farmhouse of B.V. Ashby, and completed a successful dive at the sinkhole, extending a shot line approximately into the water and reaching the primary point of interest known as the "rock pile", a central pile of limestone rubble about 40 meters directly below the sinkhole entrance. They briefly explored the perimeter of the rock pile before surfacing. With their shot line already in place, the group planned to return the next morning and continue exploring the cavern.
The group arrived at the sinkhole on the morning of 28 May after refilling their cylinders at Mount Gambier, and quickly descended to the rock pile. Reports from the divers indicate that they had not planned to explore beyond "the edge", a narrow and downward-sloping continuation of the cave on one side of the main chamber, far from the natural light shaft provided by the sinkhole opening. The rock pile was generally considered the boundary for safe recreational diving; the surrounding cavern was dark, unexplored, covered in loose silt and limestone rubble, and continued downward to depths where the effects of nitrogen narcosis become extreme if the breathing gas is not diluted by helium. Even at the depths of the rock pile perimeter, the effects of narcosis would have been significant, which may have already been impacting the judgement of the divers.
While the initial descent went smoothly, the eight divers neglected several recognised safety procedures early in the dive. The shot line did not extend all the way to the cavern floor, and was not equipped with extra air tanks, nor were the divers prepared with adequate air management strategies. The group had not established specific diving partners, and they did not use any form of safety guide line; Glen Millott later stated they did not use safety lines because eight separate lines in such a confined space would have created a dangerous situation for the divers. Robert Smith, who had dived at the sinkhole on eight previous occasions and established most of the dive plan, was not expecting the other members of the group to venture as far into the cave as they did.
Accident Separate accounts from the four surviving divers were pieced together with the help of former Chief Superintendent Wallace B. Budd to establish a timeline of what happened to the four victims. At the perimeter of the rock pile, Smith began to feel the effects of nitrogen narcosis. His depth gauge read approximately . An experienced diver, familiar with the symptoms of narcosis, Smith signaled to his group that he was returning to the top of the rock pile. The group signaled that they were going to continue exploring. Smith stayed by the rock pile, circling it for around 8 minutes while searching for animal bones. He then saw the torch of Glen Millott returning from the direction the others had gone. Glen Millott had been monitoring his air, and knew that he was out of time; he had attempted to tap Christine Millott on the arm and remind her that her time was also up, but the two were separated before he could. Smith and Glen Millott met and surfaced together to find that Larry Reynolds had already returned from his dive. Half a minute later, Peter Burr surfaced with almost no air left in his tank.
Knowing the others would have similarly low air supplies, Glen Millott quickly put on a spare tank and returned to the water. He descended to a depth of , where the cliff began to drop off. At the edge, he found Stephen L. Millott's torch and camera. Significant amounts of silt had been disturbed, and the visibility had been reduced to nearly zero. Glen Millott had no choice but to decompress and return to the surface. By the time he surfaced, an ambulance had arrived at the sinkhole. Peter Burr returned to the cavern for one more look, but found nobody. By that point, the group was aware that they were looking for bodies, and not survivors.
At the end of the planned dive, upon turning around to return to the surface, the surviving divers saw Christine Millott and Gordon Roberts attempt to ascend quickly back to the rock pile. However, instead of ascending along the slope they went down, they swam straight up, possibly because they were worried their air would be exhausted soon. The two divers swam directly upward into a dome in the ceiling which had no exit. Reynolds reported seeing their torches frantically searching for an exit before Roberts signaled back that they were lost. According to Reynolds, Christine Millott and Gordon Roberts looked "frightened." This was the last time the two were seen. Likely suffering from nitrogen narcosis, and surrounded in silt allowing minimal visibility, the two failed to find an exit. They exhausted their air supply and drowned; their bodies were later found together below the ceiling dome they had failed to escape. Reports suggest that Christine Millott and Gordon Roberts may have been holding each other, as they knew their death was imminent. Their bodies were found together.
Further down and around the same time, one surviving diver says they witnessed a member of their group swimming strongly downward further into the cave. This was John Bockerman, who was likely under the effects of severe nitrogen narcosis, and may have been unaware that he was swimming to his death. His body was located about further from where he was last seen, and was at the greatest depth of the four victims.
The fourth casualty, Stephen L. Millott, may have been seen lost beneath the cave ceiling shortly before the surviving divers surfaced. Although his torch and camera were found deep at the base of the rock pile, his body was found under an overhang at a depth of only . Compared with the other victims, little is known about Stephen L.'s final moments, but he was wearing a buoyancy vest, and because he was found under an overhang, it is likely that he was buoyant at the time of death.
Body recovery
A search of the cave by the police Underwater Recovery Squad to a depth of was initiated on 29 May 1973, the day after the accident. The search was brief, and no bodies were found. The team was aware that they were underprepared for a dive in such conditions, and the operation was described by Chief Inspector Wallace Budd of the South-Eastern Division police headquarters in Mount Gambier as a "learn as you go exercise." A second attempt was made on 30 May, again unsuccessful. The police search was ultimately postponed as the team sought Naval expertise on the dive, the training for which was expected to take several months.
The following January, the landowners permitted a television film crew, making a documentary on cave diving in the Lower South East, to enter the sinkhole. On 22 January 1974, after several days of diving in other caves in the area, the crew descended to a depth of and, using professional lighting equipment, illuminated the cave "like daylight." A technician looking in the direction of two of his teammates noticed what appeared to be a third person behind them. Further inspection revealed that it was a body in a wetsuit. The team turned the light away immediately and surfaced together to report their findings. According to the crew, the body was not filmed. On the morning of 23 January 1974, police arrived at the sinkhole and dove to a depth of , where they found a body floating under a ledge against the sloped cavern roof. The body was immediately towed out, and police divers continued searching to a depth of , finding nothing else. The body was initially only identifiable by the equipment it was wearing, and official identification required dental records. The body was found to be that of Stephen L. Millott.
The landowners, growing increasingly uncomfortable with the remaining three bodies stuck on their property, approached an amateur diving team from Melbourne to begin recovery efforts. However, the police diving team was finalising their recovery preparations around the same time.