A "secret hill bunker" could refer to a hidden or underground shelter, typically located in a hill or mountain, built for protection, secrecy, or security. These bunkers might have been used during wartime, for civil defense, or for private purposes. They are often constructed in remote or hard-to-access locations to avoid detection.

If you’re referring to a specific known location, there are several famous bunkers worldwide that might be called "secret" in popular culture or due to their historical significance. Examples include:

Cheyenne Mountain Complex (USA): An underground military facility built within Cheyenne Mountain, Colorado, designed to protect against nuclear attack and to house military command centers.
Bunker Hill (USA): The name of a hill related to the Battle of Bunker Hill in Boston, though it’s not a "bunker" in the traditional sense.
Could you provide more details about the "secret hill bunker" you're referring to, such as its location or context?