Dry sand weighs less than wet sand.   At 1.5 tonnes, a cubic metre of dry sand is the same weight as a typical family car, a hippopotamus or an average sized great white shark.    At 2 tonnes, a cubic metre of wet sand is the same weight as a white rhinoceros, a small tractor or a double cab ute. 

This photo shows what a square meter looks like.  It isn’t very big is it?  So imagine the same shape in the dune – and you have up to 2 tonnes of sand sitting above the cave.

Sand caves not only damage sand dunes, but they put lives at risk when they collapse.  Wet sand caves can give the illusion of structural integrity and therefore being safe.  However, there is no safe sand cave.  As sand dries it gets weaker and any sand cave can collapse without warning.

And, of course, when a sand cave collapses that part of the dune collapses as well. For those of us living by the beach, those fragile dunes are our first line of defence against the ocean.