What kind of a scenario would have to occur in order for human beings to seek survival on the sea floor? It could be man made or natural, ice age or heat apocalypse, sudden disaster or gradual collapse, I just want to know how it could get to that point. I'm talking about either living on the bottom of the ocean in built structures, or underneath it in tunnel systems.
Radiation, either from the sun growing unstable or from some persistent cosmic event, sleeting down from space.
A few meters of sea water will block most of the radiation that would effectively sterilize the land. People could build shelters on land but what would be the point? There's not biosphere left on land. Also, without the land biosphere buffering heat and carbon, the climate would be massively erratic, likely with super storms and possibly rapid cycling mini-ice ages at random places. Everywhere would turn into a desert devoid of life but with the same rainfall. With no life to hold soil, huge swaths of land would simply turn to mobile mud.
The ocean would also have the advantage of easy, protected mobility. Trade and migration would be safe and easy. All hell can break loose on the surface but a hundred meters down, everything is smooth.
Likely, it would start with the radiation anomaly growing over a period of years, enough to make preparations. Nuclear submarines and static basis are built. Major land resources are moved to sea accessible locations, perhaps islands. Preferably the initial refugees would be away form major continents or at least rivers because a lot of crap is going to wash into the seas. Only a few millions at most out of billions could be moved under the sea initially, but there would be survivors on land for years or decades. They could be rescued as capacity under the ocean increased.
Lots of room for story conflict there. The pioneers under the sea, likely the "best and brightest" having a life of mission but also material privilege while at the same time the survivors on land growing haggard, desperate and bitter.
The first thing that comes to mind is an incredibly large volcanic eruption. And I mean a huge eruption. Something never before seen on Earth. One of the largest (if not the largest) eruptions we know to have happened was that of the Toba supervolcano. It released a staggering 2,800 cubic kilometers of ash, and covered most of South Asia with a layer of ash 15 centimeters thick. It may have killed off a substantial portion of the human population.
But we would need something even bigger to make all of Earth's land uninhabitable, possibly a series of large volcanic eruptions caused by some massive tectonic instability. The Ring of Fire could be a significant contributor, although most of the ash could fall in the ocean. Essentially, you would need all of the world's volcanoes spewing out all of their magma - and then some. Unlikely - probably impossible - but it could still drive humans off of land.
What about that ice age you mentioned? Well, once again, we'd need to cover all of Earth's land. Is that possible? Not by the effects of recent ice ages, but the snowball Earth hypothesis has some potential. It states that, at least 60 million years ago, Earth underwent some cooling, which formed more ice, which then led to more cooling and more ice. Wikipedia suggests (just as I was thinking!) that a supervolcano could have lowered Earth's temperature just enough to start this cataclysmic cooling. There are some other obvious causes (which it also lists - perturbations in Earth's orbit, asteroid impact and an impact winter, reduction in greenhouse gases, etc.) which are a little more mundane, but equally possible. So if you can trigger any of those, you can trigger a snowball Earth.
The one man-made cause I can think of is a large nuclear war. Radiation would have to be distributed just right so that while Earth's land would become overly toxic, its oceans would be fine. Controlled bombs could produce this desired effect, but there would have to be a lot of them - many more than we have today - and they would have to be detonated in very specific spots, so as to make sure the oceans were unharmed. This would, admittedly, be nearly impossible.
Broadly speaking you need three things: (1) something that makes it infeasible to keep living on land (most people won't move unless pushed), (2) the inability to evacuate from the planet, and (3) enough time to build and move into those underwater habitats.
For (1), other answers have proposed radiation and volcanic eruptions. These work if they can be forecast far-enough in advance to allow for (3); surprise global terrorist attacks with nukes probably wouldn't allow people to move into the oceans, but a decades-long climate change or major volcanos gradually "heating up" over decades could. To radiation and volcanoes you could add massive air pollution; the logical response might be to move underground, but some will see that as just kicking the problem down the road a bit and move into the water sooner. (You're going to need oxygen and nutrients, and I suspect they're easier to get from the ocean in the long term.)
You also need to keep people on earth; this could be accomplished several ways:
- We don't have off-world colonies yet.
- We do, but we can't get most people there. (Possible story point: if we can get some there, who gets to go?)
- We have capacity but it's much more expensive to ship people to Mars/Luna/wherever than to go into the sea (so maybe the "haves" go to space and the meek inherit the earth, so to speak).
Finally, you need to be able to move people into the sea. You'll need a history of sizable underwater structures (labs, observatories, etc) already, so you're not just figuring out how to build underwater when a global war is heating up. In other words, people will build settlements under water if they already have a foundation of knowledge and engineering to build on; otherwise they will probably dig underground instead, even knowing that that has problems of its own, because we know more about digging. (And if this is set on near-future Earth, we have the established meme of underground bunkers already.)