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:
- 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.
- Hydrothermal vents or retrofitting of submarines could provide an immediate solution to power.
- Water is significantly easier to attain by desalinating the surrounding salt water, with existing technology.
- Precious metals are found scattered all along the ocean floor, and wouldn't require as much surveying & digging to find & mine.
- 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.
What apocalyptic event could realistically push mankind to pull together resources relocating to the ocean floor. Or at least a good portion of them.