How can I force humanity to retreat to the ocean floor

Question

I'm looking for an mass-extinction level event that would cause most life on the surface of Earth to be forced back to the oceans, one that is not feasibly reversible for humanity, before societal collapse sets in--but wouldnt kill mankind off too quick to start making rudimentary colonies deep along the ocean bed. My current idea is

LOSS OF THE OZONE LAYER

From what I've read, if the ozone layer were to be completely stripped away, most if not all life on Earth's surface would die out. I believe this is the most plausible reason mankind would try mass-migration to the ocean depths; however, the problem is I dont think that colonizing the ocean floor would be the first choice. Im certain subterranean habitats would be a more cost effective solution if only the ozone layer was stripped. So I need something that makes living underground just as dangerous and impractical for large populations.

My immediate thought to this would be a total nuclear war, following or preceding the collapse of the ozone layer. A summary of this paper claims a global nuclear war could cause a peak loss of 75% of Earth's ozone layer. This would provide a plausible scenario to ozone depletion and soil too dangerous to be digging around in after the fact. I'd have to do more world building for dealing with the radiation dispersed throughout the ocean, and how enough humans survive MAD to gather the resources to pursue such an endure, but I think this is a plausible baseline reason for significant populations to try inhabiting the deep ocean.

The only other solution I saw to quickly decimate the Earth's ozone layer would be a gamma ray burst stripping it away. But why humans would choose to dedicate time to going underwater instead of underground brings me back to the subterranean alternative. My immediate counterarguments for why some humans might take the plunge instead:

1. There would be a significant amount of marine-life to survive the aftermath, which colonies could readily utilize to sustain themselves instead of trying to grow crops & livestock underground.
2. Hydrothermal vents or retrofitting of submarines could provide an immediate solution to power.
3. Water is significantly easier to attain by desalinating the surrounding salt water, with existing technology.
4. Precious metals are found scattered all along the ocean floor, and wouldn't require as much surveying & digging to find & mine.
5. Technology for long-mission subs already exist for warfare; if the entirety of Earth's research and development was pushed towards advancing this technology instead of creating new tech to survive long-term underground, it could be argued as pragmatic by a politician or hyped up by an influential elon-esque figure with the ego-driven agenda of making his own society--thus bolstering public opinion even more for the high seas.

I'm up for any advice. I think my current explanation to create a setting for humanity living underwater can plausibly work. With the second iteration of it just meaning there are still pockets of humans living underground instead of underwater, and minimal or no contact existing between them--perhaps an opportunity for stories to be told in the setting.

I'd be open to even an unexplained catastrophic event, akin to the moon's explosion in Neal Stephenson's Seveneves--for the uninitiated, the raining debris superheats the atmosphere, and a pocket of humanity survives the super-heating and boiling over of Earth's surface and ocean by living in deep ocean trenches.

TO CONCLUDE

What apocalyptic event could realistically push mankind to pull together resources relocating to the ocean floor. Or at least a good portion of them.

Answer 1

I don't think you need anything exotic here. As long as the humans know in advance that this will be a long-term living situation, the ocean floor will naturally be a better choice than underground for a few key reasons.

1. You can dig an underground shelter, but you can't enlarge it after the fact. Additional excavation creates debris, but there's nowhere to put it unless you haul it to the surface (too dangerous). Seawater is relatively easy to move out of the way when you need to expand - just ask the Dutch. An underground colony would require strict limitations on population size, storage space for supplies, etc., whereas an underwater colony could expand as their needs changed.
2. Water is available underground via aquifers and wells. These sources recharge very slowly, though. Depleting an aquifer would mean certain death for the colony. An underwater colony is surrounded by a practically infinite supply of water, assuming that they could desalinate it.
3. Growing food underground is nigh impossible. Underground bunkers typically stockpile enough supplies to last through the entire disaster. That's not possible for long-term underground living. An underwater colony can raise crops on shallow portions of the sea bed, plus would have easy access to an abundance of seafood.
4. Ventilation is a big problem for underground structures. You need some way to pump fresh oxygen in from the atmosphere and to exhaust waste gases. If the big disaster left the atmosphere significantly polluted, you'd be piping this polluted air into your shelter. An underwater colony can extract oxygen from seawater via electrolysis. The water acts like a giant filter keeping the atmospheric pollutants at bay.
5. Humans don't do well in isolation. We're social creatures. Underground colonies would be completely isolated from one another, with no way to travel between them without making the dangerous trip to the surface. Underwater colonies don't have this problem, since they can just hop on the submarine bus and hitch a ride to the next town over. Aside from the psycho-social benefits, you'd also be able to have some form of trade between colonies and could better ensure the gene pool stays sufficiently diverse.
6. Power generation is likely easier underwater. The warm earth below you and the cool waters above you can fuel geothermal power plants, as can natural thermal vents. Shallower areas can use wave power or floating buoys with solar panels. A more advanced civilization can electrolyze hydrogen from seawater and use it in a fusion reactor. An underground civilization could be wired to existing solar panels but couldn't safely deploy more. You could conceivably tap into a natural gas seam and burn it for energy while it lasted, but that amount of refining and combustion underground would be extremely difficult, not to mention feed into the ventilation problem.

Underground shelters are the easy solution for short-term refuge, but only if you can stockpile everything you'll need to make it through. For sustainable, long-term living, you need the extra freedom that being underwater gives you. As long as your disaster makes the surface uninhabitable in a way that the humans can easily recognize is long-term, migration to the oceans would likely be their natural choice. Your disaster doesn't have to explicitly make underground living a bad idea, it's that way naturally.

Answer 2

Self replicating aerial drones.

Gamma ray bursts, active suns, they all have the flaw that humans could just live a bit below the surface and be fine. You need an active threat that continually hits anyone on the surface.

Rogue AIs is a great way to do that. The USA and China could both have made AIs to manage drone programs designed to level each other. Both drone programs could have been triggered, cut off any attempts to turn them off as enemy action and an endless series of drones could be made by automated factories and mines to bomb the enemy and endlessly belt patriotic propaganda into the air to convince the enemy to surrender.

Humanity will have a while to try and handle the threats, but in time the only safe place will be deep under the oceans. The AI is limited in scope, and only attacks aerial threats, ground threats, surface sea threats, and 'missile silos' aka anything underground.

Answer 3

The sky is on fire

Someone calculated that debris from the K-T extinction event asteroid would have been thrown out of the atmosphere. The IR radiation of the debris burning up in re-entry would have raised the surface temperature to around 400 degrees.

Let's say the solar system started passing through a small but dense nebula. Not dense by earth atmosphere standards, but a lot more than the near-vacuum in most nebulae. Enough to raise the temperature of the surface to over 200 degrees for, say 50 years. This kind of disaster was suggested in Iain Banks's Feersum Endjinn

Features of this disaster:

• Slow enough that mankind can react
• Overwhelming so that mankind has no chance of stopping it.
• Mankind wouldn't have to move to under the ocean, the ocean would come to them.

Why not underground? Digging takes time, and you have all sorts of tectonic issues to worry about. Just ask the people who do drilling. A few hundred meters of ocean is enough to isolate you from what's going on up there, and all it would take would be to build a bubble over your head. Ok, a really big, strong, pressure-resistant bubble, but bubbling an area could probably be done more cost efficiently than digging. It also allows you to move your habitat deeper if things get worse.

Furthermore, raising ocean levels will result in subsidence, which would increase tectonic activity globally. The only reason that silt from rivers hasn't filled up the Gulf of Mexico is because the weight of the silt keeps pushing the floor of the gulf down. Melting ice caps will do that, too.

In a real scenario, mankind would probably try both tactics. I can imagine flooding, cave-ins, people feeling trapped, and a failure to coordinate causing plenty of plot complications for the mole people.

Answer 4

Hyperbaric Disease:

A disease has swept the world, and millions have died. Everyone is affected, and even those testing negative for the disease are dying suddenly. Onset of the lethal symptoms occurs at a random time interval, so people continue to die. If nothing is done, the human trace will perish.

But a workaround is found. Hyperbaric chambers arrest the progression of the illness and delay the onset. It is impractical for millions of people to stay in hyperbaric chambers for the rest of their lives. A bold plan is proposed: colonize the deep ocean so the entire population is in constant high-pressure conditions. Everyone contributes for the chance to get in the lottery for a survivor position.

Mankind retreats to the sea, vowing to some day beat the disease and return to the surface.

Because No One Owned It:

As global warming intensifies, small oceanic nations begin to disappear under the seas. They desperately try to hold on to their existence and major countries, concerned about refugees and dwindling land resources, invest in oceanic farming and mining.

Meanwhile, the refugee crisis grows wildly and nations don't want refugees on their land. Displaced peoples are forced into floating refugee camps on the sea and the lucky ones have their labor exploited to build oceanic mines and farms to support themselves and the world.

The rich build luxury ocean communities to get away from the violent, crowded mobs of hungry people on land. A whole culture of oceanic development begins to take form. Meanwhile, humanity stresses the land to the point of ecological collapse. Wars break out everywhere.

So when your global catastrophe hits, there are all ready colonies and refugee camps in the ocean being built. They flood with incoming refugees and expand what they have as quickly as possible. There isn't time to build underground colonies. Everything else dies, and the sea colonies are on their own. They're desperate, crowded, and hungry. But they're used to it.

Because of Oxygen:

Your world is rapidly losing oxygen. Maybe an invasive airborne bacteria is consuming every bit of organic matter and converting all of it to CO2. The bacteria is unable to tolerate salt or can only survive at the very surface of the ocean. The hope is that the seas will survive and eventually replenish the air.

Meanwhile, if you want to breathe and grow things, you need to produce oxygen. The extensive network of tidal power plants means the oceans already have power and hydrolysis can make oxygen. Without water, underground facilities are exposed to the bacteria and can’t make cheap oxygen.

Answer 5

Runaway greenhouse effect

A bit too much industry and a war that smashes a lot of remote natural gas infrastructure leads to a "transition to a moist greenhouse". The temperatures go up until the oceans boil.

However, when I say "the oceans", I mean the top of the oceans. Cool water in the deepest regions may not mix very quickly, and experience at the stovetop should suggest it takes a while to boil an ocean. (I honestly have no idea how long - that paper I linked above doesn't begin to go into such considerations)

Your humans know their planet is doomed, but maybe they can ride a modified SLBM somewhere? (wouldn't bet on it)

Answer 6

Revelation 9: Monsters.

1 And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit.

2 And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit.

3 And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power.

4 And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads.

5 And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man.

6 And in those days shall men seek death, and shall not find it; and shall desire to die, and death shall flee from them.

https://www.biblegateway.com/passage/?search=Revelation%209&version=KJV

Freaking scorpions. If thats what they are, with their lady hair and lion teeth, and wings. Stinging people just because we have clean foreheads.

Those things aren't looking for people underwater. Hiding down here was an emergency plan originally because we had the underwater stuff ready. It definitely beats seeking death and desiring to die. And supposedly the scorps were only going to be stinging people topside for 5 months and then we can go back up.

But it has been 6 months already and it looks like they are still at it. Maybe "months" is like "scorpions". At least the sea cucumber farm is coming along.

Answer 7

OUR PLANET HAS BEEN IRRADIATED

Our sun, for reasons that I'll leave to you to come up with, is suddenly burning much, MUCH brighter, thoroughly bathing the planet in radiation. Anything not properly shielded by a quarter-mile of hyper-dense lead, or underwater/underground, is going to be suffering from a really intense case of radiation sickness.

A quarter-mile thick sheet of lead shielding is impractical for economic reasons, and it's easier to live at the bottom of the ocean than it is to live deep underground.

Answer 8

A massive meteor strike

Lots of things can destroy all life on the surface, but to make underground bunkers inhospitable too you need to increase the planet's seismic activity. By striking the Earth with an adequately massive meteor, not only would blot out the sun and destroy the ozone layer, but you could effectively shatter the tectonic plates into many smaller less stable plates and massively disrupt the equilibrium inside the Earth's mantle.

This would result in a time period of massive Earth quakes and volcanic activity that could crush or rip open bunkers killing people who try to flee underground. Instead those who wish to survive must go under water. Rather than living on the sea floor itself which would also experience harsh earth quakes and volcanic activity, humanity would retreat into large submarines that just stay close to the sea floor. These submarines would be equipped with seismic detection systems so if they find themselves in a place where a new volcano is about to open up under them, they can just move to somewhere else.

While a super massive meteor strike would be an instantaneous event, humanity could detect and predict the impact years or even decades in advance giving us time to build our arks.

Answer 9

Atmosphere Blown Off

There were already people living underwater as fish people. Genetic engineering had come a long way and I guess a lot of people really wanted to live as fish.

When all the air on the planet suddenly went away -- I leave this to your imagination -- only the fish people survived.

Answer 10

Because that's who survived 'The Event'

Humanity didn't retreat beneath the sea.

They died.

The surface is completely uninhabitable, nuclear war, a decades-long solar-storm that irradiated every living thing on the surface, biological weapons that de-nature in salt-water.. Take your pick.

Fortunately for the species, an experimental nigh-selfsufficient underwater colony/city survived the death of the surface-world by dint of being a mile underwater and shielded from the danger.

With the addition of surviving nuclear submarines, they have vehicles, and a lot of manpower and both nuclear power and whatever facilities the city has.

They will rebuild civilisation beneath the sea until they once-more claim the surface world..

Answer 11

Frame Challenge:

Humans were already living there, and they survived longer than anyone else.

Everyone talks about life on Mars, and fantasizes about life on Venus, but the next step for humanity is life on Europa.

Europa has an ocean of water underneath an icy crust. Living there immediately solves the four biggest problems of living in space, which are

• Where does your oxygen come from? (And what if it all leaks away?)
• Where does your water come from?
• How do you protect yourself from cosmic and solar radiation?
• How do you maintain livable temperatures long-term.

Of course, you don't go to Europa and start figuring out how to build an underwater habitat beneath an ice shelf, you perfect the technology to build an underwater habitat beneath an ice shelf and take it to Europa.

Once humans stopped talking about "living on Mars," they started building habitats to prepare for living on Europa. This would start with prefab buildings designed to reside just below the surface, and gradually move off the continental shelf. Space agencies would build progressively deeper and colder, eventually building permanent settlements under the Arctic and Antarctic ice. The final challenge is to drill down through the Antarctic ice shelf, and build a habitat from scratch.

Unfortunately, a sudden GRB apocalypse ended the whole Europa trip, along with everything else.

However, all those habitats that were built as proof-of-concepts were still inhabited by marine biologists and deep-see tourists when the gamma-ray-burst hit. Because they were constructed to allow long-term, isolated habitation, the humans there survived when surface life died off.

Answer 12

Oceans level rising too much, together with savage weather.

Your trigger event has devastated the climate of Earth and the level of oceans has risen dramatically. Simply there is no longer sufficient landmass to support viable underground settlements. Moreover the weather has gone totally bonkers. Tempests, hurricanes, hailstorms and other crazy meteorological phenomena have become common and unpredictable.

Attempts have been made to settle underground, but the current landmass is mostly made up of archipelagos and the former mountain peaks. These latter are too rocky and harsh to be colonized or excavated. The remaining plethora of smallish islands are always subjected to heavy tides and floods that can destroy most underground settlements.

Rendering an underground settlement weatherproof in those conditions is much more difficult than to retreat to deep waters, where the harsh weather conditions are severely dampened by the water mass above.

Answer 13

Lots of "Small" Meteors

A pro-Human Extinction group sends replicating drones out to the asteroid belt/Oort Cloud and starts flinging city-destroyer asteroids towards Earth targeting any large human conurbation. The impacts are enough to do a number on humanity but wouldn't do much damage to the ecosystem, particularly if they wait a month or two between each one to let the dust settle, literally. While subterranean shelters would still be relatively easy to wipe out this way unless they were so deep as to be impractical, a layer of ocean would be enough to prevent destruction with anything small enough to leave the bunnies and quokkas alive (the asteroid flingers love bunnies and quokka), so underwater humanity goes.

Answer 14

Earth becomes a rogue planet

A possibility I can think of would be that the Earth was flung out of orbit and became a rogue planet. Perhaps a black hole or other massive object passed through the solar system to cause this to happen. The surface of the oceans froze yet humanity is able to survive in colonies on the ocean floor supported by geothermal energy. Deep sea life continues to cling on as well centered around geothermal vents.

Answer 15

A relativist object hits the moon

This can scatter eject enough moon mass in low orbit, so:

1. It's a relativist object from outer space. There is no warning;

2. Causes an initial heavy bombardment causes devastation.

3. Causes a long, constant micro bombardment that turns the surface an impractical place, if not deadly.

Answer 16

Unfortunately, unbeknownst to humanity, this has already happened.

Actually this is an occult truth known by either the initiated ones or enlightened ones. The easiest example the western world an grasp the state of world is through the Matrix Movies. We live in multi-layered reality with the visible world being a tiny iceberg. Our souls came for a 'trip' on earth to test some ideas and we signed a contract before we were born. Once we leave the body and enter into the 'spirit world', we realize how vast the universe is and depending on our Karma, we may have a choice to return to The Light which is the source of All Creation. Some do reject The Light for various reasons and a significant number of humans who reject The Light join other sub-human and animal-like creatures which live in the ocean floor.

There is are mega-cities under the sea on earth but a different layer in the 'Matrix'. The creatures of this city are the agents of magic and secret societies make pacts with them. They are way more advanced then current humans and have many different competing ideological 'cults' but mostly harbor hostile anti-humanity groups who prey on unsuspecting humans and harvest organs mostly blood.NOTE: A huge number of them were of 'human' origin but mutate when they cut themselves from The Light are reject its transcendence and divinity.

Its impossible for humanity or any human to cause an apocalyptic event of sch a scale that humans relocate to the ocean floor. It is a phase of reality that was set before time and cannot be altered. And again, it has already happened and may souls live in the ocean floor in cahoots to reptilian creatures to destroy humanity.