  The wave equation is time-reversal invariant. The enclosure method using a
Neumann data generated by this invariance is introduced. The method yields the
minimum ball that is centered at a given arbitrary point and encloses an
unknown obstacle embedded in a known bounded domain from a single point on the
graph of the so-called response operator on the boundary of the domain over a
finite time interval. The occurrence of the lacuna in the solution of the free
space wave equation is positively used.
