Operation: Hidden Secret is a mod for registered Wolfenstein, made by Santeri Kiiski. It was released on June 15, 2006.
It is a classic-styled set with minor sound and code changes. Changes to the code include ceiling colors and secret level access point. 10 levels are featured, mainly of high quality.
The mod uses its own file definition, HSC.
An SDL port was released on October 19, 2013. It was ported by Andy Nonymous.