The detection of external voids can be very important in assessing the structural capacity of a buried pipe, tunnel or chamber. Voids in localized zones outside the structure create uneven loading of the structure and can allow distortion of the structure. Both of these effects can result in the failure of the structure over time. Voids can be detected using simple hammering on the wall of the structure to detect changes in the sound of the strike. Acoustic wave methods send acoustic signals into the pipe wall and record reflected waves arriving back at the instrument. Knowing the speed of travel of the wave in the wall material allows an interpretation of the wall thickness. Such methods are very well developed for some industrial uses but their application underground and in the materials used for underground piping or pipe relining systems can be less reliable even for obtaining thickness measurements. In terms of acoustic external void detection, under favorable circumstances, changes in the shape of the reflected wave and the presence of later reflected signals can offer information about whether a void is present, whether it is filled with water or air and the distance to the far side of the void. Applying the theoretical possibilities to obtain practical results in the field is under development. Other approaches for scanning for the presence of external voids include thermal methods (thermal scanning of the wall surface temperature) because the heating and cooling of a pipe wall is affected by the presence of an external void. Microgravity and nuclear-based methods also are possibilities for application but are limited in practical use.