Detonations below the surface follow a different phenomenology. When the depth of burial is quite small, the cloud characteristics are very similar to those of a surface burst. For greater depths of burial, the time sequence of different steps leading to local fallout becomes more and more dominant as a factor determining the nature and properties of the fallout, with complete containment (no fallout) occurring at a depth of burial which depends on the device yield and, to some extent, on the properties of the overburden. In order to provide a general understanding of the properties of local fallout, it is advantageous to discuss subsurface explosions and surface and near-surface explosions separately. A brief discussion of the phenomenology of the former is found also in Teller et al. (1968). During the early stages of a contained nuclear explosion, a cavity is formed containing vaporized device debris and host rock materials. The cavity wall is lined with molten rock. While the vapor cools and condenses into droplets, the molten material begins to collect at the bottom by flowing down the wall or falling from the upper portions. After a period of time, from a few seconds to a few hours, the cavity collapses and leaves a colum of brecciated (loose) material behind the chimney. Most of the fused material remains in the lower portion of what once was the cavity, but some fragments of fused rock are interdispersed with the rubble. The radioactivity is mostly concentrated in the once-molten rock. However, radionuclides that are present in the vapor State when the cavity collapses may be found throughout the rubble as well as their daughter products. Thus, the detailed distribution of radionuclides between fused rock and rubble is dependent on the timehistory of the cavity and its contents. When structural weaknesses are present in the overburden, such as an improperly stemmed emplacement hole, failure may occur with venting as a result. Since, inter alia, the cavity contents must travel a long distance to reach the surface, the vented material is highly enriched in volatile species, rare gases being a major component. Fused cavity material would essentially be absent in the fallout (and “throwout"), so that any radioactivity present (mostly 89’99Sr and !37cs, and some shorter-lived species) is in the form of surface coatings on some of the particulate matter. Thus, the fallout is highly depleted in refractory species such as 9°Zr and rare earths, as well as in transuranic elements. Complete containment (except for venting as described above) occurs when the shock wave from the detonation has degraded to a weak seismic wave upon reaching the surface. When the depth of burial is such that the initial shock is still strong when it reaches the surface, the shock wave splits into a surface wave and a reflected dilatational or rarefaction wave while the surface moves upward. As the tension wave moves deeper, the tension increases until the tensile strength of the rock is exceeded. The result is spallation of the rock into the air. When the rarefaction wave subsequently reaches the expanding cavity, a second upward acceleration phase provides additional momentum to the overburden. 551