Haloclines in Olwolgin Cave

About the site

I’ve already talked about a couple of photos I’ve taken of unique features in the Roe Plains caves, including the Black and White Raft Room and the hanging roots. Cave diving in the Roe Plains provided the biggest photographic challenge I’ve experienced thus far – taking clear pictures in a graduated halocline. Haloclines occur where salty and freshwater meet, creating a mixing layer.

Unlike the Mexican caves on the Yucatan Peninsula, the halocline in the Roe Plains caves runs at every level of the water. This means it isn’t possible to swim above it, and any diver movement through the cave mixes water of different levels of salinity. For those who haven’t dived in these conditions before, mixing the salty and less salty water results in a misty, out of focus fog. It’s pretty much the same view as you would get if you took your mask off and opened your eyes underwater in a swimming pool.

This poses some minor difficulties for sharp photography, to say the least. When you add in the coloured tint to the water in these caves, there’s a good chance the photos are going to come out looking green and out of focus. Fuzzy and green may be an accurate representation of swimming through the cave behind your buddy, but these pictures are unlikely to impress your photographic audience.

Luckily the halocline settles out fairly rapidly, and when diving through the same areas the following day there was minimal evidence of our previous visit. In some particularly silty areas, yesterday’s disturbed silt would settle into interfaces between the salinity levels, as can be seen highlighted by the off camera strobes in the second shot.

Photographic technique

My usual cave photography technique (as outlined here) involves taking the lead on the dive, and swimming along to choose an appropriate part of the passage. I then execute a neat about face, and take snaps of my approaching buddies. This method puts me in control of choosing which parts of the cave I want to shoot, captures the divers facing the camera, and allows multiple angles on the scene as the models slowly approach.

The obvious drawback is that you are taking a photograph through water that you’ve just passed through. In the ocean this may not be such a drama, but the crystal clear water of the caves is the greatest advantage for really sharp shots. In other caves some extra care in my swimming technique combined with careful exhalations in delicate areas to avoid silt drifting down from the roof had worked quite well. It wasn’t going to work here – swimming through a potential photo in Olwolgin Cave was the end of the story. What I really needed was to teleport into my photographic positions.

As such, my photographs in most of these caves were limited to areas where the tunnel or room was big enough for me to swim down one side. I would then execute my turn and start swimming slowly back along the other side, into the clear water. Staying still to take photos would allow the blurry mixing layer of water to gently envelope the front of the camera, so I quickly realised I needed to swim forward into the shot as well. The first few tries resulted in bumping noses with my buddies, but by the end of a week of solid diving we had perfected the timings and the technique.

About the photo

As you can see, the area behind the two divers is distinctly fuzzy. With Harry moving underneath and to the side of the Paul, he’s found clear water to swim into and both faces are sharp. On the other hand, the rock being lit by the off camera strobe on the right hand side is showing the effects of the fuzzy mixing layers, and the bright orange line disappears into this mirage. This is one of the few times we managed to perfect the timing and arrangement of multiple divers with multiple strobes – there are no second chances to rearrange things as the fuzzy water catches up with you.