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We are in the future in northern North America (U.S or Canada). The land has "risen" (or more accurately, filled in) with several hundred feet of earth and sand and silt so that a modern city will show only the tops of skyscrapers emerging from the otherwise-even land. The question is what might have caused this?

Additional thoughts: The cause of the cataclysm need not be a single event (nuclear war, asteroid) but rather a cascade of socio-political events starting with a catalyst (e.g. climate change leads to sea levels rising, which exacerbates tensions in the Koreas, which prompts the North Koreans to … etc.) I'll worry about the story. But the larger is issue is how and why land would rise.

ANCHORS (Set parameters for answers)

  1. We are in the future approximately 300 years, and the events that change the world happen 100 years from now (i.e. 2117 for simplicity's sake).

  2. The world is sparsely populated as a consequences of events. Enough to form sustainable communities, but the existing geo-political world of nation-states is long gone.

  3. The skies have changed and the aurora are more visible much farther south (and often) than today.

ENGINES (what drives creativity within those parameters)

A. The timeline can be adjusted but not radically.

B. A single, unlikely coincidence (but not two!) is acceptable (asteroid strikes while X is taking place, leading to …)

C. We can play with technological advances during the next 100 years to create new conditions, possibilities or circumstances.

D. [Later addition after several answers posted]. The effects need not be global. It is OK for this to be a regional phenomenon whereas other effects (but the same cause) are felt elsewhere (e.g. the tide goes up in one place as it goes down in another …)

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    $\begingroup$ China eventually manufactures absolutely everything which we (the US and Canada) import and we stop taking care of our trash in a sustainable manner. Before you know it, China has dug itself below sea level and is exacerbating a tectonic fault line in search of resources. A massive earthquake is triggered and we simply live atop a trash heap disguised as land. $\endgroup$ – MonkeyZeus Jan 5 '17 at 19:03
  • $\begingroup$ Alluvial (river) deposits might fit en.m.wikipedia.org/wiki/Alluvium especially downstream of a large dam. Glacial rebounding is too slow and wouldn't cover a city but is a real large scale phenomenon worth mentioning en.m.wikipedia.org/wiki/Post-glacial_rebound also alluvial deposits $\endgroup$ – David LeBauer Jan 6 '17 at 6:29

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Multiple, worldwide volcanic eruptions depositing ash at a rate faster than humans can remove it. This would be on an epic/global scale, but in real life, it would appear like this: Ash burying and house

Edit: Mind you, this kind of ash-deposit would be a disaster that could easily qualify as a (human) extinction-level event. All soil for crops would be buried under hundreds of feet of sterile ash - presumably globally.

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    $\begingroup$ en.wikipedia.org/wiki/Supervolcano $\endgroup$ – Catalyst Jan 5 '17 at 14:18
  • $\begingroup$ @Catalyst - I had thought of that too. The ash-rate called for would take several if not all calderas going up at once (which though cosmically-unlikely is still technically possible). Yellowstone deposited multiple feet of ash for each of its eruptions but the OP needs hundreds of feet world-wide. $\endgroup$ – Jym Jan 5 '17 at 14:25
  • $\begingroup$ I was thinking more of flood basalts: en.wikipedia.org/wiki/Flood_basalt $\endgroup$ – Catalyst Jan 5 '17 at 14:30
  • $\begingroup$ OP said filling doesn't need to be global. A big caldera could yield hundreds of feet of ash to nearer cities while causing global problems. $\endgroup$ – Pere Jan 5 '17 at 23:30
  • $\begingroup$ This does not have to happen globally. If a powerful country get destroyed, economies of the rest of the world would collapse cascadingly. Remember how Greek debt crisis shook the whole world. Following famine and chaos will kill most of the human population. $\endgroup$ – sampathsris Jan 6 '17 at 8:56
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Instead of the land rising, why doesn't the city fall?

There are many examples of sinkholes and sinking buildings.

The most famous is the tower of Pisa. But there is also a modern version in the Millennium Tower.

The tower of Pisa is cited as behaving in the following way:

The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning

So, it could be that due to changing circumstances, melting of the poles, rising tides, hotter temperature, heavier buildings, runoff sewage, bad city planning, tectonic movements, earthquakes, or even all of the above have caused a city, or some cities to start sinking. And slowly, slowly, only the tops of the tallest towers remain.

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    $\begingroup$ That's a very creative re-imagining of the problematic. I welcome continued discussion on both possibilities. Thank you. $\endgroup$ – Arwood Jan 5 '17 at 13:57
  • $\begingroup$ Major earthquakes can cause ground liquefaction which can cause previously solid ground to turn to silt in a matter of minutes. $\endgroup$ – Matt Coubrough Jan 6 '17 at 5:14
  • $\begingroup$ A lot of cities in the Midwest and on the coasts (Chicago; Saginaw, MI; San Francisco; most of Louisiana; etc) were built on swamps or other unstable land. IANA geologist, but I imagine that, given enough rain or other events that would re-liquify the ground, these cities would sink quite a bit. $\endgroup$ – thatgirldm Jan 6 '17 at 18:09
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Massive Deforestation

Northern Africa, where the Sahara desert now is, used to be lush grasslands and forests:

Most scientists believe the Sahara dried up due to a change in the Earth’s orbit, which affects solar insolation, or the amount of electromagnetic energy the Earth receives from the Sun. Or to use simpler words, insolation refers to the amount of sunlight shining down on a particular area at a certain time. It depends on factors such as the geographic location, time of day, season, landscape and local weather.

In 1999, a group of German scientists used computer simulation to create a model of the Earth’s climate thousands of years ago. They concluded that the climatic transition of the Sahara took place abruptly, within a possible span of about 300 years.

In addition, man-made deforestation can also cause desertification. North Africa used to be called the "land of continous shade", but deforestation has left it a wastland of sand.

So, a significant change in the temperature of the environment can change lush grasslands and forests into wastelands of sand.

The Sahara desert's sand is about 150 meters on average, with a max of 320 meters. Since skyscrapers are at least 100 meters tall, maxing out at 600 meters, a desertification complete with sand dunes could create the scenario you describe.

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  • $\begingroup$ Deforestation would work well with solar activity combined with climate change — before even addressing man-made contributions. Thanks for this too. $\endgroup$ – Arwood Jan 5 '17 at 18:48
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Abandonment, then desertification

No, you cannot have several hundred feet of soil / sand magically appear evenly all over the continents. That concept you need to abandon more or less immediately.

However...

Buried cities exist in real life. Especially in tropical areas we have discovered several such locations, such as these in Cambodia. And looking even closer in time, there are many fascinating abandoned places, such as Pripyat, the town that housed the workers for the Chernobyl nuclear power plant.

enter image description here

Nature is reclaiming Pripyat after all the humans left (Image source)

So the first step to start covering cities is simply to remove people. Invent any disaster you like; super-plagues, war, alien attack...

But if you then want these towns to be covered in soil or sand, then you need to destroy all of this plant matter. Desertification will do that for you. Dramatic climate changes kills the plant-life and eventually it will decay.

Do note that wind will carry some of the dead plant matter away. This means that the coverage will not been even. On great plains, the dust clouds will just keep blowing, compare the 1930's Dust Bowl. So to have this phenomena, you need cities that are in places that will capture the blowing soil. Depressions, mountains and other geological features can serve as soil traps.

In summary:

First the calamity killed the people. Abandoned cities started growing over and eventually were half-buried in plant matter. After that the great desertification happened. Some cities remained relatively unburied, but some trapped much of the migrating dust clouds and were buried.

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  • $\begingroup$ Perhaps the word "evenly" was a mistake. Visually, I'm imagining floodwaters carrying silt, receding and thereby leaving behind new land, and the cycle repeating (though in dramatic form). Yes, over time, even such a landscape would become uneven but perhaps not dramatically. This feature, though (flatness) is not as important and the bottoms of buildings being well underground. Thank you for your thoughts. $\endgroup$ – Arwood Jan 5 '17 at 14:04
  • $\begingroup$ Overgrowth will get you to a state where the bottom of buildings are buried. :) $\endgroup$ – MichaelK Jan 5 '17 at 14:06
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This is my first post, and I'm therefore a bit uncertain when to post an answer. But it's been about a month, discussion is winding down, and I am pressed on with the writing. So I thought I might share my answer:

Before providing it allow me to first say, thank you. This has been a very interesting experience and quite engaging. I'm happy to say that the following ideas would not have been possible without this discussion.

It strikes me that the moment to stop answering the question isn't when we've built a fully valid proposition, but rather when the needs of the story have been satisfied. In this case, my story is set a few hundred years into the future, and the disruptive event (the Rise) look place about 100 years from now. It seems quite reasonable as well as dramatically interesting for the characters themselves to wonder whether or not the earth rose around the buildings or whether the buildings fell into the earth (as per Inbar Rose's reflections above). If two cultures develop legends and lore around those alternative readings of the physical world, it becomes a dramatic rich point for story development. In my case, I think this works quite well.

That does not relieve the burden, however, for me to create a plausible set of conditions for the world to be in such a state (i.e. my original question). It does, however, relieve me from having to articulate it in the body of the story itself (which is not an evasion but a deference to the storytelling).

So here's what I'm thinking which will result in the world I am building but need not be mentioned in the story:

Massive solar activity disrupts communications at a politically sensitive time, thereby undermining international cooperation because conspiracy and suspicion about "what happened" overshadows scientific evidence; There is agreement on the solar flares, but beliefs hold that some countries or actors took advantage of that for gain. Global warming massively increases as cooperation ends and regional wars begin using weapons that I fear will be more in use 100 years from now (if multilateral cooperation cannot halt the proliferation) including biological and chemical weapons coupled with cyber attacks on infrastructure. Man-made pandemics with cascading consequences become very plausible. That takes care of most of the humans and many of the animals.

Deforestation does indeed take place with global warming and the solar activity and the (previous) man-made contributions results in "global weirding" (a term now used to explained the intensification of locally strange weather as a function of globally warmer weather). At that point, I feel I can reasonably stop because I can imagine (with your help above) a set of plausible conditions where a city (not all cities) face the dramatic "rise" I mentioned.

As it happens, I do think the earth will have risen (not the buildings having fallen), but the debate is of greater value to my story than the immediate answer.

A few final thoughts on details for those of you kind enough to share your ideas:

  • I think earthquake protection and higher building standards are more likely as buildings go up from now (2017) to the time of the event in at-risk areas (LA, Tokyo, Rome, Tehran, etc.). So it seems that the most modern of buildings by that time will be those most likely to survive. Consequently, most of the older buildings will indeed have fallen. So the Old World poking through might mean six or seven buildings, not 50 or 60.

    • Deforestation, desertification, changing of the quality of top soil, the movement of earth (i.e. soil) from nearby mountains down into the city's valley — all these matters could conjoin to create a plausible (local) scenario.

    • I agree that changes to ocean currents, desalination and other matters can be viewed as primary causes in the wider model.

Thank you all very much! This is connected to what I hope will be my fourth novel and I will announce its title if/when it sees the light of day.

Respectfully yours.

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Trash If current trends hold true, we are producing more trash than ever before. If it blows around, it would likely get caught in trees and among buildings. The ensuing health crisis kills off most everybody. Remaining scientists utilize cloud seeding or repurpose chemical warfare arsenals to disperse a rubbish eating micro-organism that renders the primary components to soil.

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  • $\begingroup$ The volume of trash produced by humans isn't that large even on a human scale, much less a planetary one. It only seems large because people tend to have very restrictive ideas about where it can be put. $\endgroup$ – Mark Jan 6 '17 at 2:27
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    $\begingroup$ To put some hard numbers on it, the United States produces on the order of a quarter of a cubic kilometer of trash every year. Sounds like a lot, until you realize that it would take half a century to fill the pit of the Bingham Canyon Mine. $\endgroup$ – Mark Jan 6 '17 at 2:52
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Best and most likely scenario is Yellowstone going up.

However one eruption would not create that amount of ash build up so maybe have multiple eruptions over a 100 year period and that could account for a larger layer.

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Nanobot "Grey goo" apocalypse — almost. The standard scenario is self-replicating bots go out of control and consume all matter on Earth. You just need a more limited version — they just replicate out of control, but don't consume everything. You're asking for a lot of sheer mass and volume, and you would need somewhere for that to come from — possibly automated asteroid mining. Or if you don't need it over the whole Earth, just regular mining, leaving deep pits elsewhere.

Bonus: If the material is iron from asteroids, perhaps that could alter the Earth's magnetic field enough to sound like an explanation for your aurora change idea.

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TLDR: CO2 rise -> Sea Level rise -> coastal city mostly flooded -> Humans engineered marine life that sucks CO2 out of the air super fast and stores it in shells which sink to the sea floor -> Sea floor builds up around city (made of shells e.t.c.) -> CO2 falls -> Sea level falls.


May need to up timeline by a century or two, but with sufficient acceleration of climate swings through human intervention or artistic license could be squeezed into 300 years. (No need to wait till 2117 years before the events start happening as they are happening now!)

Even if you don't believe in man-made global warming, you could for the purposes of the story or invent another cause for it...

Full Timeline:

  • Global warming, ice caps melt at ever accelerating rate, Humans, in the effort to prevent global warming, genetically engineer a phytoplankton that grows much faster than normal and sheds lots of calcium containing shells (see Coccolithophores and chalk formation). (Causing Coccolithophore blooms has already been suggested as a way to prevent CO2 rise, minus the genetic engineering bit).

  • Sea level rises massively anyway, swamping a coastal city (perhaps over 50m rise if all ice on earth melted). (Climate always lags behind CO2 anyway so dropping CO2 now may still not prevent further short term warming, so very plausible).

  • A small river that used to flow through the city is now bigger due to a shift in local weather patterns (very plausible given world climate shift) and water erosion of previously arid lands containing lots of loose soil leads to the river dumping tonnes of silt into the sea over the city.

  • The water around the city will be super nutrient rich from all the silt, this would feed the genetically engineered algae's growth massively, shells are dropped around the city with the silt, speeding up its burrial.

  • The action of the algae worldwide does lead to CO2 depletion, eventually the temperature catches up with the CO2 levels (even lower than before the human caused spike) enough for icecaps to begin forming again.

  • In the far future, this cooling could lead to a snowball earth (if the algae removed enough CO2) but for your setting there could be similar weather conditions to the ones we have now (perhaps even slightly cooler).

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I already have an answer listed, but thought of another that combines with Inbar Rose's. Inbar said the buildings sink. I started to think of what could cause the ground to be unstable or at lease less supportive of the weight of buildings. I've heard of this being used.

Vibration

Due to directed sonic attack or prolonged aftermath, the building structures moved in a harmonic resonance that worked the building deeper into the ground as if it were in quicksand. Depending on the depth to bedrock, some buildings would sink lower than others.

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Alternative/additional scenario to the deforestation/desertification one above: a significant (say 100m?) drop in the ocean level (e.g. the caverns in the "Journey to the center of the Earth" filled through a crack? Or just nuclear fracking in some place gone terribly wrong?) lets lots of silt/sand to be carried by wind in the form of massive coastal dunes.

Info on the total amount of water - water is about 0.001% of Earth by volume; insignificant percentage, reasons for water going underground may be easy to imagine/justify.

See an example of a town swallowed by coastal dunes and the story featuring rabbits and cats; feel free to add other potential causes for accelerated desertification.

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continuous sandstorms and windstorms after a global water reduction. There could be numerous reasons for such but massive shifts of earth blown around in a multitude of storms over time. The earth and sand moved would then compact and sedentarily set over more time to form a more hardened surface. Just a thought.

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  • $\begingroup$ Welcome to Worldbuilding! This is your first answer, isn't it? :-) If you can, take some time to back up your answer with analysis and math. Could storms really move that much dirt in 200 years? Would the skyscrapers have survived storms of that intensity? Try to get away from "just a thought" and move toward "this would definitely work." Answers that are less wishy-washy are most helpful to storytellers. $\endgroup$ – SRM Jan 8 '17 at 6:18
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This could be caused by an Expanding Earth. Originally this was hypothesized as the explanation for the phenomena that would be later identified as due to continental drift.

The expanding Earth or growing Earth hypothesis asserts that the position and relative movement of continents is at least partially due to the volume of Earth increasing.

Source: Expanding Earth

If instead of this being a discredited geological hypothesis, what if this expanding Earth turned out to be real. Then the land rise would be a natural result of the expanding Earth in the post-apocalyptic future.

The Expanding Earth theory isn't a joke, this was a genuine scientific hypothesis. The author of this answer had the strange experience of sitting in a lecture given one of the major proponents of this theory, S. Warren Carey, the Professor of Geology at the University of Tasmania. Since this is Worldbuilding, there's no reason why the concept can't be used here.

There are 3 forms of the expanding earth hypothesis.

  1. Earth's mass has remained constant, and thus the gravitational pull at the surface has decreased over time.
  2. Earth's mass has grown with the volume in such a way that the surface gravity has remained constant.
  3. Earth's gravity at its surface has increased over time, in line with its hypothesized growing mass and volume.

In the real world, the Expanding Earth theory was discredited.

The hypothesis had never developed a plausible and verifiable mechanism of action.[10] During the 1960s, the theory of plate tectonics—initially based on the assumption that Earth's size remains constant, and relating the subduction zones to burying of lithosphere at a scale comparable to seafloor spreading[10]—became the accepted explanation in the Earth Sciences.

The scientific community finds that significant evidence contradicts the Expanding Earth theory, and that evidence used in support of it is better explained by plate tectonics:

Source: ibid.

If the Earth was expanding, then three hundred years in the future land both locally and globally could have risen.

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To have a city barely peaking out from the top of sand or soil you would have to move a lot of earth and anything that moves that much earth/ash/sand unfortunately would be devastating to all life on the planet. Our earth is over due for its next ice age. All it take is global warming to continue to cause catastrophic scenarios.

Delicate systems and currents around the world would be destroyed. Too much fresh water in the oceans not only causes the water levels to rise, but it also devastates the natural jet streams that keeps the currents moving. When this system is disrupted it also effects the wind currents and weather. More water means more rain, and a tendency for things to cool down.

Now that all of this water has risen say, 50-80(your choice) feet you now have New York City underwater. We undergo the ice age and New York is now a frozen block of ice. Snow piles up and eventually it becomes a frozen wasteland. The Poles creep down the planet and soon enough the only reasonably habitable places are closer to the equator, Florida would have weather closer to what New York or Canada has now. Florida would also be immensely smaller. It would snow in places it has never snowed before. This would be your best option and most likely scenario that could occur.

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If you're looking at coastal real estate then there are a couple of solutions 1. consider that if we manage to melt the West Antarctic and the Greenland ice sheets then the sea will rise about 80 feet which would then lead to large-scale deposition of sediment across what are today coastal cities, if we then managed to reverse the temperature build up the ocean recedes and those buried cities are back on the coast. 2. cataclysmic cooling like the movie The Day After Tomorrow would result in large-scale sand dunes blowing inland from the exposed continental shelf which could easily bury cities in a number of areas.

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an asteroid impact triggers numerous sub tectonic activities. Landmasses rise high above the other.

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    $\begingroup$ This answer doesn't provide much information, would you care to expand on it? You can edit your answer to add more information and flesh it out. $\endgroup$ – kingledion Jan 5 '17 at 13:45

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