  After formulating the pressure wave equation in half-space minus a crack with
a zero Neumann condition on the top plane, we introduce a related inverse
problem. That inverse problem consists of identifying the crack and the unknown
forcing term on that crack from overdetermined boundary data on a relatively
open set of the top plane. This inverse problem is not uniquely solvable unless
some additional assumption is made. However, we show that we can differentiate
two cracks $\Gamma_1$ and $\Gamma_2$ under the assumption that $\RR^3 \setminus
\overline{\Gamma_1\cup \Gamma_2}$ is connected. If that is not the case we
provide counterexamples that demonstrate non-uniqueness, even if $\Gamma_1$ and
$\Gamma_2$ are smooth and "almost" flat. Finally, we show in the case where
$\RR^3 \setminus \overline{\Gamma_1\cup \Gamma_2}$ is not necessarily connected
that after excluding a discrete set of frequencies, $\Gamma_1$ and $\Gamma_2$
can again be differentiated from overdetermined boundary data.
