The key to the source of the atomic bomb’s energy was provided in 1905  by Albert Einstein. One of the implications of Einstein’s theory was that matter and energy are interchangeable with one another.

In other words, a mass (m) can be converted into an amount of energy (E), where c is the speed of light. Because c is a large number and thus c squared is huge, a small amount of matter can be converted into an awesome amount of energy.