quantum mechanics - Is it true that no one, not even Planck himself, knows how Planck's Constant was arrived at?

Planck's Constant is said to be one of the smallest constants used in Physics. Some even say it is one of the imponderables – something that we know exists and is true but cannot be explained yet. I remember reading somewhere that Max Planck somehow discovered this constant but was unable to explain how he did so. While experts may dismiss it as a concept of quantum mechanics, a branch of science that is still under intense research, a common man might see it as nothing short of a mystery.