system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40K] what is the hidden knowledge that the guardians won’t allow adapted mechanics to access on mars?
gpt
The hidden knowledge is essentially a triple-layered shitstorm.

At the surface level, they are protecting the "remains" of the Dragon of Mars, both from prying eyes and from those that would seek to free it or use it to their own ends. Knowledge that a C'tan, one of the most powerful and dangerous natural beings we know of in the Galaxy, is sitting under what is easily one of the single most important installation in the Imperium, second only to the Golden Throne, itself, would be pretty damaging information.

In addition to that, though, they are hiding an even more dangerous secret; The Emperor, himself, put the Dragon there. Nobody knows exactly when, but earlier in Humanity's life, long before we had left our own solar system, The Emperor, not yet revealed to us all, fought the Dragon and somehow imprisoned it on Mars. Knowledge that our lord and savior not only allows the C'tan to remain there, but that he actually put it there, himself, knowing what it was, would be a PR nightmare.

The real secret they are protecting, though, is much more dangerous; Not only is there a C'tan under Mars, and not only did the Emperor put it there, but a lot of technological innovation that would later assure our ascension to and domination of the stars was developed and reverse engineered directly from the Dragon, itself. Knowledge that our Emperor, one who says that all Xenos are dangerous scum that must be eradicated, who says that all Xenos technology is a direct affront to everything that the Imperium stands and fights for, who says that Humanity must forge its own path on its own terms, actually used a Xenos life-form, and Xenos tech, to benefit Humanity could legitimately lead to a second civil war.