We present a case history for the location of a cavity in the subsurface by processing the signal generated inside the cavity by a seismic source. The signal emitted by the source is recorded by surface sensors and it is processed in order to obtain direct arrival traveltimes. The traveltimes are the input of a Bayesian inversion procedure for the evaluation of the source location and of the velocity model: the results are the estimated values and their associated uncertainty. We propose a parameterization of the problem when absolute times are not available. The probabilistic approach helps the analysis of the conditioning problem, and it can aid the definition of the acquisition geometry, as a function of the source signal, the transmission medium, and the precision of the solution. The location results are accurate in the limit of seismic resolution, even in a rough environment. Other applications of the procedure are the rescue of survivors below ruins, the monitoring of underground tunnels, the positioning of the bit in trenchless excavations, and the location of microseismic events.