Conclusions (reached by a relatively small group meeting in conjunction with the Second Symposium on Protective Construction held at RAND, March 24-27, 1959) on the present status and the probable future of research into the underground phenomena associated with air- or surface-burst nuclear explosions. While emphasis in the main symposium was on construction techniques and practical problems, this small working group was principally concerned with the physical phenomena related to the survival or failure of deep underground structures. The range of pertinent phenomena discussed extended from the earliest phases of cratering through the propagation of various waves and included the interaction of these waves with underground structures.