A small puzzle.

Description from my old video on the game:
The Hidden Below is a first person shooter. It was developed and published by German company Soft Enterprises, with distribution by Kingsoft. You take the role of a private investigator who sneaks into a factory during his investigation of a case of industrial espionage. However, it turns out that vicious aliens reside inside the factory, and so you have to fight your way through all floors right to the top to get rid of all the aliens.

The game has somehow a place in history due to the features of its engine. It was released in 1994, shortly before Doom II, but already featured slopes, the ability to look up and down, the ability to jump, crouching, diagonal elements and Duke Nukem 3D-stlye security monitors.
Try to never miss the map device.

In terms of gameplay, the game plays a bit slower than other shooters at the time. Enemies are fewer and you need to find an item in each of the spacious levels, so you do more exploring. This gives the game a small hint of Action-Adventure