When atoms merge into a heavier element. This generates energy across the electromagnetic spectrum. Until 1987, it was generally thought that this could only occur at extremely high temperatures typical of plasmas. Thus, an atomic fission explosion was deemed necessary to generate sufficient heat to produce a fusion explosion in hydrogen bombs. Attempts to fully control high-temperature fusion reactions have, as of this writing, universally failed, primarily due to borderline phenomena related to containment of the reactions. See Cold Fusion.