About the site
One Tree Sinkhole is located to the south east of Mt Gambier, near Little Blue and other sinkholes in the area. Unlike the crystal clear visibility of Kilsby’s Sinkhole, it can be quite murky. The shallow layers of water are often warmer and have a greenish tinge from the algal growth. After breaking through into the deeper, colder and hopefully clearer water, sunlight only filters down very dimly.
One Tree has a classic sinkhole formation with the shallowest area being the middle of the hole, dropping steeply away to deeper areas around the edge. The natural rockpile in the middle has been augmented by a variety of farm implements and other items disposed of over the years. The most famous one is the harvester pictured above. We also encountered old rope, 44 gallon drums, sheep skeletons and bare, twisted tree trunks. Unlike ship’s wreckage found in the ocean, the still and cold freshwater here preserves the remains and makes for an interesting dive.
About the dive
The water level in One Tree varies seasonally and with the local climate conditions, and it can be a challenging entry. On this occasion the water was high enough to stand knee deep in the water to finish gearing up, then step out and splash into the sinkhole. We descended through a layer of murk that cleared up at about 12m, and the bottom appeared below us.
My recollections from diving this site four or five years ago were of a bottom at 30m+, so it was a pleasant surprise to find the harvester on the floor in 27m. The fine silt is easily disturbed, so we moved slowly to surround the harvester and I lined up some shots. From the harvester we moved on around the slope past the rope you can see in the second shot, and out to the more natural rock walls.
About the photo
My main aim on this dive was to get a nice shot of this harvester, as a slightly more unusual example of underwater wreckage. The parallel and perpendicular lines of the machinery line up well for photography and create great shadows under strobe light. The usual underwater photographic mantra is that you should get low and shoot up. Despite the sunlight being too weak for the camera to pick up, shooting from a low angle gives more negative space to place the diver in, while outlining the arms of the harvester against the black water.
While fiddling with settings, I noticed the halo effect I was getting from the off camera strobe. Caused by backscatter from tiny particles in the water, it’s similar to the strobe flare in McCavity Cave. By holding the strobe behind himself and pointing directly backwards rather than at an angle, Nat has outlined his profile in blue light. The cooler blue temperature of the Inon Z240 light contrasts with the red wood of the harvester, which is only edge lit by the on camera strobes. Most importantly, a halo is a great way to outline a dark diver against a black background.
Pitch black backgrounds are normally only seen in macro underwater photography, not wide angle. The unique characteristics of this sinkhole allow both the diver and the harvester to really stand out of the darkness.