They never found it.
We can be fairly sure of this because, today, we know that there are side effects to large amounts of one element turning into another. And one of these is a nuclear explosion:
"I HAVE IT!! The secret to - oh shi-"
In fact, today, we can top that anyway. Because today we can turn light into matter. Y'see the highest energy form of light is called gamma rays. And, since matter and light are actually the same thing in different forms*, gamma rays can sometimes make the jump into becoming material particles. For the rest of the explanation I'll hand you over to a professional explainer-of-things, or 'teacher'.....
* I skipped over that a bit fast. It's like this: Everything in the universe is actually made of this stuff called 'mass-energy'. Mass-energy can act in a way that we recognise as light, or it can act in a way that we recognise as matter. It depends on how much of it you have, what's happening to it, and there are complicated rules telling it when to act like matter or like light. But it's all just mass-energy in a different disguise.