  When an object is dropped into a bed of fine, loosely packed sand, a
surprisingly energetic jet shoots out of the bed. In this work we study the
effect that boundaries have on the granular jet formation. We did this by (i)
decreasing the depth of the sand bed and (ii) reducing the container diameter
to only a few ball diameters. These confinements change the behavior of the
ball inside the bed, the void collapse, and the resulting jet height and shape.
We map the parameter space of impact with Froude number, ambient pressure, and
container dimensions as parameters. From these results we propose a new
explanation for the thick-thin structure of the jet reported by several groups
[J.R.Royer \textit{et al}., Nature Phys. \textbf{1}, 164 (2005)], [G.Caballero
\textit{et al}., Phys. Rev. Lett. \textbf{99}, 018001 (2007)] and [J.O.
Marston, \textit{et al}., Physics of Fluids \textbf{20}, 023301 (2008)].
