Hive World
Hive is the Imperium's pet name for its Arcologies, cities on the scale of mountains. Once a world gets one, it ends up building more until it is more-or-less covered in hives, and we call the resultant mass of metal and humanity a Hive World. If you're not in one of the towers at the top, they're generally very shitty places to live, what with the gang-infested wretched hives of scum and villainy, starvation, hobos due to lack of resources, forcible conscription for the Imperial Guard, and the occasional mutant tribes in the Underhive sewers, and such. The only reason the Imperium keeps them around is because they make great recruiting grounds for the Guard and the Adeptus Astartes. Yes, you read that right. A Hive World is on par with Feral and Death Worlds for recruiting Space Marines.
You know that intro blurb? "To be a man in such times is to be one amongst untold billions. It is to live in the cruelest and most bloody regime imaginable." Yeah, that's life on a Hive World. Grimdark to the max.
The moral of the story is that organized urban planning initiatives should NOT be taken lightly.
Hives tend to get old, worn with constant reconstruction, and eventually very confusing. As time goes on the lower levels invariably start to cave in on themselves due to the ever increasing weight from above and the decay that sets in on ill-used regions. This causes the phenomenon of the 'underhive', a mess of forgotten and collapsing old tunnels and buildings that becomes what is basically Detroit on steroids: a Ghetto where the poorest, gangs, mutants, freaky life forms of all sorts and pollution tend to collect. In worse and older hives the underhive can be lethally toxic, radioactive, and house everything from mutant gangs with pet giant death-roaches to malfunctioning servitors armed with power scourges to even the occasional Genestealer. Needless to say these are wonderful places to visit if you happen to be looking for a quick death, but for rogue psykers and other people with no choice but down they are generally home for however long their lives last.
As a very loose upside, the underhive functions as a flush-valve for the dregs of society, a clear area to keep an eye on and fortify against, and something to focus the masses on hating rather than letting them hate upwards. On the further downside it's still a breeding ground for dangerous beasts, insane cultists (sometimes of the Genestealer variety), psykers, gangs, plagues, and worse, and getting down there to repair anything is a daunting if not impossible task for anything less well equipped than an entire arbites unit complete with armor support...... and in older hives, it might even be utterly impossible for anything shy of a kitted out inquisitorial strike team or squadron of space marines!
Aside from all the other dangers, there are also hivequakes, which is what happens when part of the lower levels settles or breaks down further and things above it are affected. This can be localized, widespread, or even catastrophic if the piece that finally gives out was bracing things that were holding up chains of supports that lead to collapses much further above. They can also lead to invasions of mutants and renegades, especially if the quake opens up a sizable rift into the heart of the hive where the underhivers can loot valuables and necessities. Thus in many hives hivequakes are as common as weather pattern shifts and contribute to the sense of paranoia that already comes with living in an overpopulated spire stuffed with billions of religiously hateful, often violent, or outright criminal people only barely controlled by watchful and well armed police forces.... if that much. And you thought your neighborhood was bad! On the other hand, the people being invaded by the underhive during quakes are also well-armed, far more numerous, and fanatical zealots screaming at the top of their lungs. It's more dangerous for the underhivers, really, but desperation is despseration.
Oh, and the underhivers are every bit as fanatically devoted to the Emperor as anyone else in the Imperium (hell, even pirates are fanatics).
As mentioned above, those who can live in hives are good at navigating crowds, familiar with a certain level of tech, and often very alert, all traits that make them good soldiers, inquisitorial fodder, or marine potentials (depending on their individual aptitudes). Humans who grow up in deep hives or lower down, like Ciaphas Cain tend to have pretty good senses of direction and spacial visualization, as well as navigate the dark pretty well. Hives also concentrate humanity into one location and provide them with tools and space to work, meaning massive amounts of labor gets done in a hive by a never ending tide of workers, so production is essentially nonstop. This is, of course, one of or even the worst case of a plot hole in all of 40k. Because if the Imperium has so much production going on, why the fuck are the Imperial Guard so poorly equipped? There is no reason. It would mean that the Hives, the Imperial Worlds, and the Forge Worlds all devote only a minuscule fraction of their production to the Guard. Which makes no sense given the Imperium's situation (and hatred of aliens). They're being pushed back world-by-world and yet they aren't even trying to devote any notable amount of their production to the military? Bullshit. Consider the following: In 40k, the IoM is also devoting resources to Adeptus Mechanicus unit construction, Space Marine armor and weapons making, tank production, all of which do not compare to something like Titans, Knights, or such. As well, superheavies also take time, and so does manufacturing ammunition. All this means that the IG gets the short end of the stick. They are also intended to be expendable, so in any case, giving them more complex gear would be bad.
See Also
- Armageddon, a Hive World and site of three wars.
- Cthonia, a Hive World and homeworld of the Luna Wolves/Sons of Horus (subjected to Exterminatus after the Horus Heresy).
- Necromunda, a Hive World and the setting of the tactical skirmish game of the same name.
- Nostramo, a Hive World that served as the homeworld of the Night Lords (subjected to Exterminatus by Konrad Curze).
- Praetoria, a hive world that probably looks like something out of a Dickensian novel or a poem by Rudyard Kipling.
- Terra, the Hive World to end all Hive Worlds, as humanity was born there. Or rather Ecumenopolis, since Hive Worlds usually have(badly polluted) wilderness areas but Terra is one planet sized city like Coruscant.
- Zayth, a Hive world with a massive super continent that is bombarded with war, that instead of well placed trenches and fortifications like on Krieg, the city moves...
- Forge World, another sort of world, dominated by manufacturing and research rather than residential construction.
- Stalinvast