So I have been planning to do one of those outworlds that has a formicarium inside of it. Y'know? Anyhow. I want to make the formicarium out of grout. And the surface of the outworld out of soil. But I want to make sure my ants don't dig in the soil. Any suggestion on how to keep them from digging in the soil? Here is a picture of what kind of Formicarium-in-Outworld looks like just in case you don't know.
BUT WAIT! that's NOT ALL! I know you are looking at this and thinking "Well there's your anwser, just make a thin layer of soil above the grout" well I can't use too much grout because it is expensive BUT. I can use a decent amount to make like 2-3 inches think of grout. so enough to make decently sized tunnels. But I still have a decent amount of space inside the rest of the outworld. Any ideas?