  We present evidence for the interactions between a ball and the container
boundaries, as well as between two balls, that are mediated by the granular
medium during impact cratering. The presence of the bottom boundary affects the
final penetration depth only for low drop heights with shallow filling, in
which case, surprisingly, the penetration becomes deeper. By contrast the
presence of the side wall causes less penetration and also an effective
repulsion. Repulsion is also found for two balls dropped side-by-side.
