It rises like some impossible jewel above the cool sands of the Barren Sea, a massive dome of metal and silicates glistening in the too-white light of Acheron. It is testament to all the emotions felt by the first generation of colonists upon the arid world of Tarterus, the shock and despair that they had been stranded, a single orbital path from the world they had been promised, their ship a wreck that would never return to the skies; and then the determination that they would survive regardless, and that what had been lost would not be forgotten. The Atrium is the most extravagant and lavish structure on the planet, a low dome standing a mere 150 meters in height, but covering nearly ten kilometers in width, sectioned off into large habitats. Within, plants and animals imported with them as seeds and embryos form a carefully structured array of ecosystems, a snapshot of the living green and blue of Earth, utterly alien to the arid world beyond the walls. It is a shrine to long-lost Earth, a monument to the beauty of life, and a testament to the determination of Mankind in the face of adversity.
When the colonists of the Noah awoke from their timeless cryonic sleep to find nearly half their number dead, large portions of their equipment lost or ruined, and their supposed ark of a new civilization broken on the landscape of an arid world, a single orbit out from the pelagic world they had been intended to colonize, it was almost more than any of them could accept; to be so close to the promised water-rich Eden they had envisioned when joining the colony effort, and yet unable to reach it seemed a cosmic joke at their expense.
Had they been a straight sampling of the human populace from Earth, they might well have been overwhelmed by it. They had been carefully screened, however, selected for resilience and psychological fortitude. Within a year, the mood had shifted from disbelief and despair to determination and energy. If they could not have their promised land, they would instead claim this dry world, and one day their children would lay claim to the blue dot in the sky they now named Elysium, even as they named their new world Tartarus.
That they would not forget this promise, they began to lay plans and foundations for a great structure, possibly, they thought, the greatest the planet would ever see. It would be a testament to the world they had left behind, and a promise of the world their descendants would someday claim. A place of clear, running water, luxurious growth, and wild animals, open to all who would wish to wander it. Even with the help of the technology that had survived the landing, and most of the colonists donating their time freely to the project, it took decades to complete and seal sufficiently to keep the waters from bleeding away. The first generation of Tartarans was well into young adulthood before they could see what the world their parents had forsaken looked like; yet, to those first-born, it was like a miracle, a gift so poignant that they swore to make the dream of their parents a reality.
So it is that the Atrium has stood through each generation since, never maligned by the factions that have risen and struggled across the dry landscape, even after Elysium’s colonization by tenth-generation inhabitants. Even further into the future, it yet stands, even as the skip-shuttles surf the curve of Acheron’s gravity well at superluminal speeds, linking the system’s worlds, moons, and stations in a dense societal web. It is the First Wonder of the World, a special place where anyone is welcome, from the swarthy inhabitants of the mining facilities of Moloch to the settlements on frozen Cocytus, and on out to the frigid stations dug into massive Kuiper objects, shipping ice and volatiles inward to the warmer worlds.
The Atrium, to all, is a tribute to the uniting elements of all Humanity.
-A strange plague is moving through the domes. A chance mutation? An accident? Or has someone finally decided that they are not of the Humanity that built the Atrium?
-A band of fanatical political dissidents have moved to seize the Atrium, threatening to destroy it if their demands are not met; will their demands be met? Will they truly destroy the Atrium if not? Can they be stopped without damaging the building or agreeing to dangerous deals?
-An infamous figure from the outer stations, known for advocating breakaway policies, has announced his desire to visit the Atrium; under common law and social precedent, he must be allowed to do so in peace. Is he up to mischief? Will one of his enemies try to strike at him? Perhaps he needs some guards, such as the PCs, who may or may not agree with his political views. What happens when he begins an impassioned speech to them, and his views actually make sense?
-Some people want the Atrium expanded, to become a testament to all life on the worlds mankind is known to have touched. Political factions are vying wildly over this proposal. If given a go-ahead, PCs might be tagged as biota collectors, or saboteurs, or guards, or contracted to haul construction materials from the Kuiper colonies. They may have to hunt down some hostile Elysium predator that escapes into the frightening and uncomfortable landscape of Tarterus. Or perhaps something goes wrong, and they have to go into the expanded Atrium to find out why panicked calls for help are being broadcast.