Have you tried 'Sink hole!'? This ELI is a classroom demonstration of sink hole processes and their effects.
Sink holes commonly form over limestone bedrock, but also develop where the underlying materials are even more soluble, such as rock salt and gypsum deposits. This classroom activity models the dissolving of underground materials and the impacts this can have at the surface.
Many more good teaching ideas on a wide variety of topics can be found on the website.