Cavity Detection

What possible technologies to be used in detecting cavities in a fully-solid shaft?

Let’s say that we have a solid shaft of any type of material, and due to imperfect environment, the shaft somehow is now have cavities that we cannot observe with just eye inspection.

Is there already a technology to provide such inspection or we have to improvise?

This article has discussed some crack detection technologies. The 6 most important methods for crack testing in non-destructive material testing