Because of the fallout, the test site must be isolated. The presence of human population does not improve nature (with exceptions which are quite rare and the more notable). Also, to keep the site clean, tests must be carried out in the absence of rain. Therefore, at the site one usually finds sunshine and solitude. For the participants the beauty of nature forms the back-drop to preparations of experiments which are difficult and exciting to everyone involved. At the end, the atomic explosion is always dwarfed by its setting. But the work that culminates in the detonation is rewarded by something quite different from a flash and a bang. The really important results of a test consist in marks on photographic plates. Most of the apparatus that produced the plates has been destroyed in the explosion. But enough is saved so that one can conclude what has happened in the short fractions of a second that pass between the pressing of the button and the knowledge in the observer: this was it. In those fractions of a second another stone was added to the structure which we may call astrophysical engineering. What happens and what is observed in nuclear explosions are closely related to the behavior of matter in the interiors of the stars. The details of the nuclear explosion cannot be described here for three reasons. First, the details are secret. Second, the size of this book and the forbearance of the reader set limitations. And third, we understand only a small part of the process. Within these limitations, this is what happens: The actual nuclear reaction takes only a fraction of a microsecond (one microsecond = one millionth of a second). All the energy of the bomb is released in this short period. At the end of this period, the main body of the nuclear material is moving apart at a rapid rate and by this motion further nuclear reactions are stopped. In addition to the more or less orderly outward motion, considerable portions of the energy are found in the disorderly temperature motion, which has stripped most of the electrons off the nuclei and has transformed the atoms into a freely and chaotically moving assembly of charged particles. By this time many of the original nuclei have been transformed into nuclei of radioactive species, partly by the fission process and partly by the capture of neutrons in all sorts of atoms which had been originally present in the bomb materials. Still another portion of the energy is present as electromagnetic radiation. This radiation closely resembles light except that it is of shorter wave length and is therefore not actually visible; but it can be absorbed and re-emitted by all sorts of materials, and is in a violent exchange of energy with the exploded bomb fragments.