Experiments are described in which mild steel plates of 1/10; in. thickness were set on hardened and ground flat steel dies having central square-shouldered round holes, of diameter ranging from 1/10; to 1£ in., and subjected to the shock wave resulting from the detonation of an underwater high explosive charge. Graphical results are presented showing the variation of the deformation of the plates due to cupping and to shear, with increasing charge size, and with varying hole diameter.