[AstralTraveller (David)]

Looking at the sinkhole I'm struck by just how circular it is. I think other sinkholes I've seen have also been round (there's a nice one south of Braidwood).

Now the underground cavern is long and sinous and the fissures through the rock are almost certainly not straight and vertical. So why does the rock failure have such a regular shape? Why isn't it elongated and irregular? The answer is obviously more in physics and engineering rather than earth science.