What would a post-apocalyptic underground city be like after 1,000 years?

Question

The city is modern/futuristic and is fairly large. It was designed to be self-contained, and has been sealed shut for a thousand years. All human residents died, so there has been no maintenance. There are living beings (monsters/demons as well as things like rodents, insects etc.). There are also things like plants, water, etc. There is no light or heat (although I could conceivable include a fictional way to provide those things, such as mutated fungi or magical light sources).

Specifically, I'm wondering what would happen to vehicles, signage, windows, electronics, buildings, etc.

Answer 1

With no light, the plants all died quite quickly (it takes a lot of light to keep plants growing), and the animals died of starvation. The bacteria, fungi, etc., will have exhausted the formerly-living materials and died too. There might be viable plant seeds, fungal spores, and the like, but nothing is actively alive.

If your demons needed food to stay "alive," they'll be gone. If they only ate for the sake of cruelty, they'll be really keen to do that. If they were trapped in the city, they may well have destroyed anything that could be useful out of frustration at the absence of any other outlet for cruelty.

Answer 2

Vehicles - completely non-functional, rusty chunks of ore. But some plastic parts will survive just fine. So it will be more like a blobs of iron and zinc ore with cool plastic parts inside. Sponge of the seats will desintegrate, plastic hard panels of the front and doors will remain intact but somewhat brittle. Lots of plastic in engine parts and fuel system will remain fully intact. Rubber wheels will become very brittle, but will mostly hold its shape. Plastic tanks and pipes can be used, if new clips are used, and pipes are not bent, as they will become brittle too. Plastic tanks remain intact. Gasoline and diesel will become waxy and unusable for a car. Water may remain in a screenwash tank. Radiator system will rust away and lose coolant. Oil will be mixed with motor's rusty remains.

Road signage - somewhat functional, metal including the pole and tension ring will rust away and fall off. Plastic will remain as is, probably on the ground nearby, but will be covered in dust. Careful wiping of the dust can reveal bright colors as today. Plastic of this type will still likely be brittle, unlike some car parts.

Windows - Somewhat functional. Glass is intact. Inner gas pocket seal is broken, so they provide no thermal insulation. Inner aluminium frame will oxidize, but outer PVC frame will stay. Hinges will oxidize. Windows will stay in place, but wont keep warmth, and will fall off as a whole unit when touched. Window will be covered in dust and not let the light through. When fall it will shatter completely into small chunks, as PVC loses its flexibility additives, glass shatters normally, and oxidized aluminium is dust anyway.

Electronics - completely non functional. component leads (legs) will rust away. Copper surface will oxidize and become green. Solder may stay intact. Plastic will stay intact - all the element shells. Elements' remains will remain in place, but without any attachment to the board will easily fall off when touched. Plastic parts and ferrites will stay intact, but not that useful without leads or the board. Ferrite cores could be used. Long pieces of copper wire will remain okay-ish, as only ends will oxidize. So ferrite core and copper wire could be used to wind up a transformer.

buildings - many will stay functional. Skyscrapers will fall because of steel rusting away. One story wooden houses will fall because wood is eaten away. But dumb and ugly 5-9 story concrete blocky buildings are likely to stay as is, it is just concrete and it is not affected. Old stone buildings like churches also have a good chance to stay as is, losing its wooden roof but remaining mostly functional.

Overall everything will be covered in a sticky grey dust, no bright colors to see when flashlight is turned on unless dust is wiped. No transparent things either. No complex plants either, more like moss and fungi. Most common entity that survived will be advertising objects: plastic cards, plastic 'paper', plastic signs outside the business, plastic emblems, price signs, plastic packaging and labels for goods, door numbers, cheap plastic chairs and toys.

Glowing mushrooms is a thing, and can grow there too. No need for those, but they look cool.

Some insects may survive eating what remained of those who decomposed the wood.

Some reptiles could theoretically survive at extremely slow metabolic rate, but this is pushing the limit.

Warm bloded animals cant survive on such a low metabolic rate. So no birds, no mammals.

Could replace rodents with large spiders to keep the 'ewww', but also not throw away the realism. They wont be fast though. Movement is restricted to 1 cm/s at most, likely even less, due to metabolic rate limitation. Faster creatures will starve to death.

Chemotrophs are likely to dominate in such conditions. In particular eating metals and rubber. Basically a goo, microbial mat, that covers everything that has water and sulphur or organic or metals. Probably thats how the life began, and it will end in a similar way.

Answer 3

First, temperature will be constant: it'll be whatever the temperature of the surrounding rock is. Near the surface, this will approximate the annual average of the ambient air temperature (50-60 F in most of the continental United States); then the deeper you are, the warmer it gets because of the Earth's hot interior.

Paint, dyes, and such will likely last better than they would with light, as light (especially ultraviolet) is a primary source of fading for dyes and organic pigments. You won't have plants as we usually think of them without light, but there could be fungus, and caves have their own populations of insects, fish, reptiles, and amphibians adapted to not needing eyes (a thousand years isn't long enough for rats to completely lose their sight, though).

If the environment is dry enough, wood and metal will be preserved better than on the surface, but that isn't likely the case if there's life present, at that requires and distributes moisture. Also, rodents, insects, and fungi will tend to break down natural materials, so wood frame structures are likely to be brightly colored ruins after this much time.

Generally, anything electronic will be unserviceable after more than a couple decades -- there aren't any circuits of any complexity that don't depend on electrolytic capacitors, and these go bad when not used for as little as ten years. This is why a desktop computer can continue to run continuously for ten years or more (running the same software it got when it was new), but if you get one from an abandoned office building, there's a good likelihood it won't start up. The same is true of radios, cameras and flash units, cars new enough to have electronic engine controls, and so forth. Insulation seems to be tasty for some rodents, so electrical wiring may have issues, and of course car batteries will be unserviceable after as little as a few months of neglect.

Answer 4

To maintain animals, you need plants. To maintain plants, you need warmth, light, or ideally both.

Your demons and monsters are unlikely to take a side salad with their meals, so they would probably prey on the largest non-monstrous creature in the area. If a monster eats like a carnivorous human, let's say each monster needs 50 pigs a year, one per week, ish. Supporting large animals like that requires some serious calories to be available.

To this end, I would suggest making your city home to geothermal power, which fuels the growth of dark-leafed plants , mosses, and lichens that survived 99% of the lights going out. Maybe that's why that location was chosen for the city in the first place? It's not unreasonable to think that these near-future people could gene-splice together crops tailored to grow underground; imagine a field of wheat, or an apple tree, almost jet black. Over a thousand years of neglect, basically everything broke down, including the heat transfer conduits, meaning a large number of physically disparate locations within are overrun with plant and animal life, all supported by heat not going where it was designed to go. And as a bonus, this provides light in some places, in the form of dully glowing metal.

So, with all that out of the way: vehicles would be rusted to unrecognisable scrap, and completely devoid of paint. Ditto signage. Windows would probably be alright, unless the heat's really something to write home about in some places, in which case they would likely have broken from either sustained heating or repeated heating and cooling, depending on how static the environment is. Note that suspended windows, like the wall panels of large public buildings such as art galleries, are held in place using metal fasteners that will definitely have given way. Electronic circuitry is a non-starter, as others have said, but it's always possible to fix some stuff with Handwavium technology.

Buildings are a little more tricky. Some concrete mixtures will last centuries, some will be crumbling in fifteen years. Some buildings don't even use concrete anymore, or if they do, only in the foundations. An underground city doesn't sound like something that would spring up organically, which implies it was a designed habitat, which implies that it would have been put together all around the same time, or at least sufficiently close enough together that after 1000 years it makes no difference. The key, then, is consistency: decide how unstable and crumbled your buildings are, and stick to it (at least by category; steel-framed buildings would definitely have collapsed, long ago, and rusted into almost nothing).

Personally, I'd go a little unusual and flatten your city a bit, make it so the wildlife and passage of time have made it into a semi-open plain, with concrete pebbles and dead things forming a semi-toxic soil underfoot, dotted with outcroppings in the form of old buildings. It's not quite a maze, it's too open for that, but it's certainly not somewhere your average line of sight is very long. If you want to gain access to any of the old buildings, you'll need to bring seismic imaging equipment and a shovel.