The Underhive is where Omin ends and the planet begins. Below the manufactorums, beneath the thermal shafts, past the last hab-blocks and the final vestiges of Imperial control, the hive gives way to a realm of natural caverns, ancient tunnels, and geological phenomena that predate humanity's arrival on Sethis Major by millions of years. This is not a place meant for human habitation. It is hostile, lethal, and utterly indifferent to the struggles of those who venture into its depths.
To descend into the Underhive is to leave civilization behind entirely. There are no Arbites here, no Tech-Priests maintaining infrastructure, no Lords Leeran issuing edicts. The Imperium's authority does not reach this far, and those who come here do so for their own reasons: desperation, greed, exile, or something darker. Some seek rare minerals and heat sources that can be harvested and sold. Others are fugitives, hiding from crimes committed in the levels above. A few are explorers, driven by curiosity or madness to map tunnels that wind endlessly into the dark.
The Underhive is not uniform. It is a chaotic labyrinth of chambers and passages, some carved by ancient geological processes, others excavated by the Adeptus Mechanicus in the early days of colonization and then abandoned when the work became too dangerous or unprofitable. The temperature varies wildly—some areas are scorching hot, heated by proximity to Sethis Major's molten core; others are freezing cold, their rock walls coated with ice that never melts. The air is often toxic, filled with radioactive particles, volcanic gases, and the byproducts of industrial runoff from the hive above.
Life in the Underhive is a brutal proposition. Those who dwell here—and there are some, though their numbers are small—exist on the absolute edge of survival. They scavenge, they hunt, they carve out territories in the tunnels and defend them with savage desperation. Some have been transformed by their environment, their bodies twisted by radiation and chemical exposure into something no longer quite human. Others have retained their humanity but lost something else—sanity, morality, hope.
But the Underhive is not empty of value. Deep beneath Omin's foundations lie resources that cannot be found anywhere else in the system: rare minerals, geothermal energy, water sources untouched by the hive's pollution, and stranger things that the Adeptus Mechanicus would pay handsomely to acquire. For those brave or desperate enough to venture into the depths, fortunes can be made. Most who try will die. But some will return, their pockets heavy with wealth and their minds scarred by what they have seen.
The Underhive is also a place of secrets. Ancient structures have been discovered in the deepest caverns—buildings that predate the Imperium, carved from stone in styles no Tech-Priest can identify. Strange symbols mark the walls, and those who stare at them too long report headaches, visions, and worse. The Adeptus Mechanicus has sealed off several of these sites, declaring them forbidden and posting guards to ensure no one ventures near. But the Underhive is vast, and there are always more tunnels to explore, more chambers to discover.
For the citizens of Omin, the Underhive is a myth, a story told to frighten children and remind them of their place. But for those who live and work on the edge—the scavengers, the prospectors, the cultists and criminals—the Underhive is very real. It is a frontier, a proving ground, and a graveyard in equal measure.
The Sethis Major Exploration Group—universally known by its unfortunate acronym, SMEG—is a resource extraction company that operates in the Underhive under contract with the Adeptus Mechanicus. The company's primary mission is to locate and harvest valuable resources from the planet's depths: rare minerals, geothermal energy, water, and anything else that can be extracted and sold. SMEG also works to open and maintain "safe" heat tunnels—passages that allow controlled access to the thermal energy radiating from Sethis Major's core without incinerating anyone who ventures near.
The SMEG Outpost is the company's main base of operations in the Underhive, a fortified complex built into a large natural cavern at the boundary between the inhabited Lower Hive and the wild depths below. The outpost is a sprawling collection of prefabricated structures, storage depots, equipment yards, and worker barracks, all surrounded by reinforced walls and guarded by private security forces. It is the last bastion of order before the chaos of the Underhive proper, and it serves as a staging ground for expeditions, a marketplace for scavenged goods, and a refuge for those who need somewhere to rest before venturing deeper.
The outpost is always busy. Prospectors come and go at all hours, hauling cartloads of minerals and salvage. Engineers monitor the heat tunnels, adjusting vents and coolant systems to prevent catastrophic failures. Security teams patrol the perimeter, watching for threats from the depths—gangers, mutants, or worse. And in the heart of the outpost, the company's administrators process contracts, file reports, and manage the logistics of an operation that is equal parts profitable and insane.
SMEG is officially a private company, registered with the Administratum and operating under the oversight of the Adeptus Mechanicus. In practice, it is secretly owned by Rogue Trader Borgnine Taxis, a shrewd and amoral businessman who has built a small fortune by exploiting resources in places too dangerous or remote for the Imperium's official apparatus to bother with. Taxis does not advertise his ownership—it is better for business if SMEG appears to be an independent operation—but those in the know understand where the real power lies.
What Taxis does not know—or perhaps chooses not to know—is that SMEG has been infiltrated by a genestealer cult. The cult is small, no more than a few dozen members, but they are well-placed within the company's hierarchy. They work quietly, slowly, spreading their influence and waiting for the day when the Patriarch calls them to action. Some hold positions in the outpost's security teams, others work as engineers and prospectors, and a few have burrowed into the administrative staff, where they can manipulate records and divert resources.
The cult's presence is a ticking time bomb. If the Inquisition or the Adeptus Mechanicus discovers the infestation, the consequences will be catastrophic—not just for SMEG, but for the entire Underhive. But for now, the cultists remain hidden, their alien masters lurking somewhere in the depths, waiting for the right moment to strike.