Almost as impressive as the development of the nuclear bomb was the near simultaneous development of cameras fast enough to record the initial events of the nuclear explosion:
[youtube KQp1ox-SdRI]
Two phenomena worthy of mention:
1) The “mottling” of the early fireball is due, in simplest terms, to differences in the density of “stuff” around the actual nuclear bomb. The casing of the bomb, batteries, control systems, cables, support structures, all played a role.
2) The “spikes” below the fireballs, something that became known as a “rope trick.” These were only visible on nuclear tests where the bomb was mounted on a tower. While the fireballs expanded outwards at extremely high velocity, the expansion speed was nowhere near the speed of light. And in this case, the light preceding the fireballs was bright enough to cause the steel guy wires stabilizing the towers to evaporate and superheat into incandescent plasma. The light alone was enough to cause solid steel to explode with more force than any high explosive.