There is no noble gas known as "The Hidden." It's likely a misunderstanding or a fictional concept.

Here's why:

* Noble gases are well-known: All the noble gases (Helium, Neon, Argon, Krypton, Xenon, Radon) are well-documented and studied.

* No hidden element: The periodic table is organized and complete. There are no undiscovered elements that are commonly referred to as "The Hidden."

It's possible you're thinking of a specific story or concept where this name is used.