WARNING: The following contains spoilers for S.W.O.R.D. #7 from Marvel Comics, on sale now.
The X-Men have been pushing the boundaries of the Marvel Universe, with the formation of Krakoa and the conversion of Mars into Planet Arakko only being some of their most dramatically open actions. Perhaps, more importantly, is their development of Mysterium, a substance that has almost infinite value and potential in the universe, and only accessible thanks to the cooperation of the Six.
S.W.O.R.D. #7 by Al Ewing, Stefano Caselli, Protobunker's Fer Sifuentes-Sujo, and VC's Ariana Maher reveals that S.W.O.R.D. has been pulling Mysterium from the White Hot Room -- meaning the X-Men have found a way to pull resources from one of Marvel's most ethereal and dramatically powerful dimensions.
The White Hot Room is an otherworldly dimension with deep connections to the material world -- and it turns out the X-Men and their allies have been effectively mining it. During his dinner with Storm on Planet Arakko (formerly known as Mars), Doctor Doom ends up revealing he's been studying Mysterium, the powerful new substance discovered by the Six. Thanks to his experience with the mystical side of the world, Doom has been able to identify its unique elements, including the fact that it may even be capable of resisting and even countering magical forces. Calling it "the Mystery" and the "Above-Place," it turns out the Six have been collecting the energy of the White Hot Room, primal kirbons of energy that can then be converted into the highly durable and adaptable metal.
Reaching into that realm is a MAJOR advancement for the mutant-race. The White Hot Room is one of the most mysterious aspects of the Marvel Universe. The plane of existence exists outside of the core universe and is connected to the material universe via a handful of cosmic-level connections -- such as the M'Krann Crystal. Notably, the White Hot Room has long been linked to the Phoenix Force. A universal force of chaos and rebirth, the Phoenix Force is one of Marvel's most powerful entities.
It's been suggested that the White Hot Room is both the home of the Phoenix Force, a realm between life and death, and even the true heart of the Phoenix itself. Only a handful of beings have ever been able to witness and return, typically those with some deep connection to the Phoenix such as Jean Grey and Quentin Quire. Within that realm, the various other hosts of the Phoenix Force, past and future, can be witnessed -- as Jean did when she saw the realm in the New X-Men storyline "Here Comes Tomorrow" by Grant Morrison and Marc Silvestri. It's a strange and unknowable place, and one of the few true mysteries left within the Marvel Universe.
This is likely why Mysterium is such a powerful substance and goes a long way towards explaining the full potential of the substance. If it's from the White Hot Room, then it's truly of another caliber than most other substances. The fact that it can change from a flexible metal to one of the strongest substances in the universe with ease speaks to how, like the realm it originates from, it can easily change from one thing to another. It's also, as Doom compares it to, similar to stealing "fire from the gods." This is cosmic level stuff of the utmost degree, with the Six apparently finding a way to collect -- and even weaponize -- the building blocks of the realm that exists after death. Positioned as the opposite of the Below-Place, it's clear that Mysterium is even more potentially powerful and devastating than anyone realized.