A dense and toxic atmosphere makes Ohmsworld inimical to human life. None visit the world or choose to live within its countless small hive spires because of the hideous environment and the dramatic weather.
Instead, most residents are the descendants of human colonists who were compelled to live upon the planet due to its unusual mineral assets. This is because the atmospheric contaminants make Ohmsworld a valuable holding. Through careful atmosphere filtration, the inhabitants can obtain several chemicals and compounds key in the creation of Adamantium.
There are few records of Ohmsworld's initial colonisation. The filtration towers represent one of the greatest technological marvels within the Severan Dominate, but their ultimate origins are unknown. The technology is clearly consistent with Adeptus Mechanicus design, but there are no records of its creation outside of those jealously guarded by the Machine Cult. It may be that this world was colonised before the arrival of the Angevin Crusade in what would become the Calixis Sector.
Recently, Astra Militarum forces began a significant push to secure Ohmsworld. This is in part an effort to secure the world's ore production, but also an effort to keep that same production from the Severan Dominate. The important caveat is that both the Secessionists and the Loyalists believe the hives are far too precious to risk damaging.
To date, most of the war's efforts have been battles conducted on open plains far from any hives, within the murk of the planet's toxic atmosphere. The few exceptions are instances where agents were able to penetrate a hive's security, so that Loyalist forces could enter the structures, fighting the resistance in brutal, room-to-room combat.
Nearly all of Ohmsworld's human population dwell among the countless hive spires that dot the planet's surface. Though each is far less populous than a typical Imperial hive city, many of these delicate spires are every bit as tall and nearly as broad. This is largely because the enormous structures are little more than air filtration systems. A vast network of delicate tendrils, constructed of near microscopic tendrils, which resemble minute feathers, filter the world's atmosphere.
As air constantly cycles past and through each hive city, the filters carefully identify and isolate the most valuable of the rare ores. The immeasurable numbers of microscopic tendrils capture flecks of each component—far smaller than a grain of sand. The filtration devices pass these precious elements to collection systems. The ore is then smelted into larger bars, which can be transported to off-world refineries and manufactoria.
As a side effect of the filtering, breathable air is directed to each hive's interior. This is an absolutely crucial effect, as without it, the world's inhabitants would be entirely dependent upon rebreathers. Oxygen is simply too rare for filtration plugs or gasmasks to suffice. Most citizens who venture forth from the hives wear tanks of breathable atmosphere, as any extended foray upon the planet's surface is otherwise too dangerous.
The situation is so extreme that there are no known populations of humans -- or even mutants -- dwelling on the world, aside from those within the hives. Due to the world's inherently toxic atmosphere, agents of the Adeptus Mechanicus have failed to identify any native life forms. They hypothesise that nothing dependent upon a carbon-oxygen life cycle could survive within the environment. If any creatures live within the roiling atmosphere -- as local legends suggest -- it might be a creature whose biology is completely foreign to descendants of Terra.
- Only War: Core Rulebook (RPG), pp. 346-348
- Only War: Enemies of the Imperium (RPG), pp. 8-31