Albert Einstein, noted by Time Magazine as "Person of the Century," was the first to comprehend that energy and mass (matter) are equivalent. When he wrote his world-changing paper ("Does the Inertia of a Body Depend Upon its Energy Content?"), the 26-year-old patent clerk was so forward-thinking he had no prior sources to cite. His paper, as originally published, was without footnotes.
In a startling departure from prior thinking, Einstein theorized (E=mc2 is his famous formula) that matter (the physical "stuff" in the universe) could become energy (that which matter needs to move) and, conversely, energy could become matter under the right circumstances. As he himself explained (follow the link to hear him):
...very small amounts of mass may be converted
into a very large amount of energy and vice versa.
Once Einstein’s revolutionary theory was accepted - which did not happen overnight - scientists realized that enormous amounts of energy could be produced from splitting tiny particles of matter - like atoms. (For submarines, such a power source would later transform “boats that dived” into true sub-marine vessels which could remain underwater indefinitely.)
But how would such an energy transformation actually occur? How was it possible to "open" an atom?