Easter eggs and secrets can be found not only in relatively recent releases, but also in classic games of a respectable age. A developer with the nickname memory_fallen discovered a cheat code for Doom that remained unnoticed for 27 years.
The hidden function was found in the version of Doom for Sega Saturn, which was released in 1997. The port turned out to be quite problematic and was not popular, so it is not surprising that no one could find the cheat code for so long.
You can activate it with the following set of keys: X, Right, B, Y, X, Right, B, Y. After that, the walls on the levels will become translucent. As memory_fallen specified, the same cheat code is in the PlayStation version called X-Ray Vision.
Earlier, the enthusiast launched voxel Doom on a 3D display. Before that, a major graphics mod was released for the second part of the shooter, which added support for ray tracing.