In order to explain the process of forming the outer rings of an impact crater, a model of an underground explosion of an impactor penetrating into the target is proposed. Intense crushing of the surface rocks is explained by the effect of the primary compression wave if it propagates from a buried source (a camouflage explosion) and when reaching the surface, causes a rapid expansion of compressed rocks at the free boundary (forming the so-called "spalling" cracks). The author's calculations show that the formation of ring spall cracks can occur at all diameters that are multiples of the diameter D of the crater that is visible on the surface (as it is observed in real multi-ring structures) if the compression wave length λ fits an integer number of times (n) on its propagation path up to the first rings on the Earths surface. For the expression obtained under these conditions for λ(n,D), it is possible to calculate the depth of a source H(n,D) and to estimate a minimum depth of possible penetration of the impactor as the explosion source Hmin = 0:975D.