Blowing potholes

As part of our trip southwards last week, we visited one of the many caves scattered across the Margaret River region. The immediate impression on entering the 'Jewel Cave' is its vast size. It's hard to estimate just how big the main cavern is, but as a rough guess maybe 100 metres by 50 metres by 10 or so metres high – probably higher in places. The guide told us we had walked nearly a kilometre on the tour and climbed up and down 500 steps as part of it. There's a lot of volume to it. 

The cave was discovered only relatively recently, in the 1950's (from my memory of what the guide said). What drew people's attention to something special was the 'blow hole' on the surface. There is only one natural way into the cave, through a pot hole that's conveniently (for the cavers) wide enough to get a person down, but not much wider. The original explorers had to lower themselves tens of metres down the pothole and then through the cavern, before they touched the ground. We, on the other hand, entered through a man-made tunnel in the side.

This pot hole used to (until the new entrance was built) blow out air or suck in air as the atmospheric pressure changed. A sudden drop in atmospheric pressure outside, for example, would create a pressure differential between the inside and outside of the cave, and the cave would expel air. With a vast volume inside and a pretty tiny hole to come out of, a small shift in pressure can create an intense flow of air at the pothole. it was this extreme flow of air in and out that suggested there was something very big down there, and that this hole was possibly the only way in.

A surprise was also how dry the cave was. The Margaret River region is not a dry area of Australia by any means, but there wasn't a drop of water visible. In fact, the water level has dropped considerably over the last 30 years,without any obvious reason. The cave shows evidence of large changes in water level throughout its history, but why is unclear. There are some hypotheses, such as the lack of a large bushfire above the cave in the last 30 years leading to more leaf litter than might be normal, but the reality is that this is an open research question. There's a lot of science to do here.

Oh, and the biologists can get excited too because there have been thylacine remains found here (Where a human can squeeze, so could an unobservant thylacine).

There's a lot more to this cave than meets the eye.





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