The detonation of an explosive charge underwater results in an initial high-velocity shockwave through the water, in movement or displacement of the water itself and in the formation of a high-pressure bubble of high-temperature gas. This bubble expands rapidly until it either vents to the surface or until its internal pressure is exceeded by that of the water surrounding it. (The volumetric expansion of the bubble also leads to a drop in internal temperature in accordance with Charles’ Law.)
At this point, as noted above, the overexpanded bubble collapses into itself, leading again to a rise in bubble pressure and internal temperature until such time as the bubble pressure exceeds water pressure. The bubble again expands, although to a rather smaller size. A second shockwave is produced by this expansion, although it will be less intense and of rather greater duration than the first. With each cycle, the bubble moves upwards until it eventually vents or dissipates into a mass of smaller bubbles.
The number of cycles, while generally low, is difficult to predict; they and the overall effects, depend on explosion depth (and thus water pressure)
, the size and nature of the explosive charge and the presence, composition and distance of reflecting surfaces such as the seabed, surface, thermoclines, etc.
This phenomenon has been extensively used in antiship warhead design since an underwater explosion (particularly one underneath a hull)
can produce greater damage than an above-surface one of the same explosive size. Initial damage to a target will be caused by the first shockwave; this damage will be amplified by the subsequent physical movement of water and by the repeated secondary shockwaves or bubble pulse. Additionally, charge detonation away from the target can result in damage over a larger hull area.