The Cemeteries of the Petrol Age
Prior to the Cosmic Era, at the height of the Petroleum Era, the world population broke through the 10 billion number, and cities grew like a cancer across the planet, upwards and outwards. The sprawls created were impressive, hundred of square miles of paved roads, parking garages, high rises, and high density residential zones and overflowing suburbs. These suburbs would eventually urbanize, and new suburbs would grow out from them. Then the Second Dark Age fell across the globe, and with the Population Contraction, in a century the population bubble burst, seeing the world empty out.
The cities that had built arcologies or had access to immediate alternate power sources, like hybridized or alternate fueled nuclear reactors were able to survive. Those many conurbations that lived off the support of the grid and pure infrastructure fell. Without power, without local industry or agriculture, the survivors went other places. The legacy of the Petroleum Era is the hundreds of abandoned, ruined cities, left in it's passing.
The Dead Zones
The official dead zones are Black Contamination Zones, areas that have become environmental hazards and are dangerous enough that they cannot be inhabited and are too dangerous for even clean up crews to work in. Scavenger operations sometimes are carried out in dead zones, but this is rare. It does little good to risk one's life to bring back valuable materials to find them radioactive or otherwise contaminated. The most common type of Dead Zone is caused by radioactive contamination, either from fall out from detonated nuclear weapons, or from compromised nuclear reactors. These dead zones are typically marked by now rusted signs and decayed barricades demarcating the safe perimeter of the zone.
The biggest bombs, the ones designed to annihilate entire metro areas, to shatter mountains and flatten forests, those bombs were never used. There is a thing to be thankful for, I guess. The biggest warheads were the first ones taken out of service by the global powers to have their fuel confiscated for the power reactors to use. When the nuclear exchanges occurred it was small scale and localized. The superpowers of the age didn't nuke each other. Pakistan and India, Israel and Iran, the nukes were thrown between nations with centuries of hatred between them, and in the end their hatred destroyed most of them. Most of the nukes detonated were small, and were detonated by terrorists, subversives and other factions. The American cities that were hit with bombs were hit by Americans, likewise with the European and Chinese cities. Small dirty bombs were used, a few blocks at most of physical damage, but radioactive dust clouds spread for miles. It was the sort of thing that people were terrified of.
Radiation: Traveling through a radioactive black zone requires protective gear, or the operation of a remote vehicle such as an android surrogate or remote piloted robot. Even with protective gear, humans in hot zones will require decontamination, anti-radiation drugs, and potentially genetic therapy to undo the damage done by exposure. The damaged condition of the urban areas make this a dangerous proposition, even before adding radiation into the equation.
It is easy for the modern person to forget that the Petroleum Age was not technologically backwards, the citizens of America and the European Union, China and other nations had computers, and the first and second iterations of interconnected networks, they had what are perceived to be primitive nuclear weapons, but they also had already pioneered viral weaponry by the beginning of the 21st century. During the Resource Wars, a small number of viral weapons were deployed and used. Most of these operations remain classified, as even centuries after the exchanges, some of the viral contamination zones are still hot. With the right equipment, samples, and time, a modern nation or corporation could recreate the Viral weapons that proved more devastating than atomic and nuclear weapons. We know this because it's already been done twice. Our records indicate that 6 specific viral weapons were used, and we have their profiles. Since then, two of those profiles have been detected in modern epidemics, rest assured that there are operations in place to find who was potentially behind this.
Biological: Biologically contaminated zones require the same precautions as working in a radioactive location, but have the advantage of generally being more easily decontaminated. The inherent danger of biological and viral weaponry is that many strains were designed to resist normal decontamination techniques, or have epidemiologies that lend themselves to being spread. Testing for a certain viral agent requires knowledge of said viral agent, and with the digitization of information at the middle and after portions of the Petroleum Age, much of this information is simply gone.
Not all listed Dead Zones are actually Dead Zones. With the well deserved fear and caution around such locations, simply marking an area a Dead Zone is typically a good way to keep everyone away from it. As such, there are a number of modern military and clandestine facilities that have been built and concealed in normal urban ruins and then marked as Dead Zones. These bases, bunkers, and research facilities are top level security, and black budget operations.
According to official records, the Petrol era city of Rochester NY, was struck by a series of low yield atomic weapons, detonated by terrorists who would later go on to become part of Amerikka Command, and is listed as a Dead Zone. From a distance, the city is in a state of decay consistent with being abandoned for several hundred years. This is not the case, as the Atlantic Federation maintains a base under Rochester, an enigmatic facility only known as Andromeda. The Andromeda Facility houses the Federation's Dimensional Biology research center, and as such is part laboratory, part Special Containment Procedure facility, and part prison. The facility is currently holding a number of people contaminated or mutated by Dimensional Fatigue Events, extra-dimensional entities, and even a hekatonkheire.
Urban Primitives and the Reservation
There are a small number of people who have since decided that they want no part of the Cosmic Era, and it's lifestyle and values. These anachronistic individuals have found that by moving into the ruins of dead cities they are largely left alone. This has created a urban primitive culture in a number of ruined cities. It is a Lord of the Flies/Post apocalyptic culture, primitive, violent, and barbaric. Body modification through piercings, tattooing and scarification are common.
The urban primitives that have developed their own unique culture are given special protections, and the areas they inhabit are given the same classification as Dead Zones. The urban Reservations are derogatorily called 'wholes' from the shorthand HOL, or Human Occupied Landfill. Anthropologists and sociologists study these emergent tribes, while geneticists study their blood and tissue samples to observe their biological condition.
There are some cities that have largely survived since the Petroleum Era, such as London, New York, Paris, Brasilia, Johannesburg, and others. These cities retain their massive sprawls, with large sections of the cities abandoned. These sections do not have working utilities, the buildings are in varying states of decay, and form the outer layer of the city. These are frequently called dead zones, not because of any contamination, but because there are currently no plans to remove the ruined structures (time and cost versus the salvageable value of the building) and no plans to restore them and their municipal services.
Lower case dead zones are frequently occupied by criminal elements, and those who have been exiled from the city cores for various reasons. It is not uncommon to see walls and other barriers erected to separate dead zones from the urban rim and favelas that surround many cities. For most people, being forced into a dead zone is the end of the line. This applies to androids and sentient machines as well, as they have almost no access to power supplies, in much the same that most people have limited access to food or clean water.
Practical Applications of Dead Zones
There are a number of practical uses for Dead Zones: Sense of Scale, Hives of scum and villainy, cool fight scenes.
In the Sense of Scale, the number and size of the Dead Zones can illustrate that despite the accomplishments of the Cosmic Era, the world is a much smaller, much emptier place than it used to be. For every denizen of the Cosmic Era, there were ten denizens of the Petroleum Era. This is compounded by the fact that in the Cosmic Era, arcologies have very high population densities, as well as a sizable percentages occupying subterranean geofronts, underwater seacologies, floating raft cities, space stations, and colonies on Mars, Venus, and a number of moons throughout the solar system.
Dead zones that aren't horrifically contaminated areas are hives of scum and villainy, because those are the sorts of people and machines that survive the harsh conditions. There are resources, there is food, water, and power, and it's all a game of who can get what, and what they can get it for, or what they can get for it. These zones can also house a number of undisclosed parapsychics, discarded veterans, outcasts, criminals, and the sort of professionals who have no place in the bright lights of the cities. And mutants.
Dead Zones make for cool fight scenes because there is no reason for the fighters to hold back their heavy weapons fire, there are plenty of buildings to destroy, and lots of landscape material to use in an improvised fight scene. Super powered parapsychic martial artists can use their earth shattering punches and tornado kicks. Power armor troopers, cyborgs, bounty hunters and gadgeteers can unleash their compliment of weapons and turn freestanding objects into melee weapons with impunity.
There are plenty of forgotten places in the Dead Zones, the cities were decentralized, and grew haphazardly. There can be data archives and lost server farms found in strange places, caches and arsenals of lost technology weaponry, and covert ops things that were dropped and forgotten. This can range from the innocent, such as crashing into a CDC biologic containment unit, or subversive such as finding a first gen Seed AI dormant in a Department of Defense mainframe that has been offline for centuries. The denizens of the Cosmic era forget that their technology was born in the Petroleum Era, like the supercomputers, magnetic weaponry, and prototype automated fighting machines.