casting > object of empty space

I made a drawing of a rock with a hole eroded from its center. (Through Rock) After working with this particular rock for over a month, I became more and more intrigued with its hole - and the idea of casting the empty shape. How does one cast nothing? Could I find a way to translate the time and events that created this perfect shape into something tangible? I used clay, latex, and resin with varying results. Here is a record of some of the attempts and some of the results.

2016
2016
2016
2016
2016
2016
2016
2016
2016