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A part of the story I am writing involves two continents that centuries before the story takes place sunk into the sea.

I want to have a way to explain this that does not come down to just the sea level rising high enough to swallow those continents, but rather the continents ''sinking'' as it were. Preferably without them ripping apart a la ''2012, Doomsday, Apocalypse, Atlantis'' type scenarios.

Is there any non magic way an entire continent could ''sink'' like this in a pretty rapid fashion?

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    $\begingroup$ Even a sinking rate of centimetres per year pretty much has to be catastrophic. The Earth just isn't that malleable. If you "continents" were really broad flat islands relatively close to the sea level in the first place then you might get away with it, but you'd still me more likely to end up with swamps and vast expanses of reef and sandy keys than open ocean. How sunk do you want? $\endgroup$ – Starfish Prime Oct 16 at 14:34
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    $\begingroup$ Maybe you could put it down to insects? Wearing away at the lower layers of the continents until they collapse? It probably wouldn’t work, otherwise I would put it in an answer. $\endgroup$ – Snorka Oct 16 at 15:07
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    $\begingroup$ No, there isn't a non-magic way for continents to sink rapidly. Continents basically exist because they're composed of rock that's lighter than the oceanic crust. $\endgroup$ – jamesqf Oct 16 at 17:20
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Introduce a moon.

Without a moon, the planet won't have any tides, so the sea level won't experience any short cycles. Add a moon, and you suddenly have variations in water height which on Earth can be as much as 15m. With a bigger, more massive moon you could easily induce even larger tides, enough to flood the majority of a low-lying continent. Note that you will need quite a long tidal period to allow full continent-wide flooding, which you can achieve either by having a very slow rotation period for the planet (ie a very long day) or a very fast lunar orbit period, so the planet is almost tidally locked to the moon.

The challenge, then, is simply to introduce a moon over the timespan of a few centuries in a way which doesn't cause horrific devastation to the biosphere. Depending on your genre, The Aliens Did It may be an acceptable explanation. Otherwise, your planet needs to capture a wandering moonlet and shed its excess orbital momentum in a way which doesn't result in a planetary bombardment of moon fragments. Exercise for the reader.

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    $\begingroup$ Alternatively, the story could take place on a moon of a much larger gas giant. While moons tend to be tidally locked, which will prevent changing tides, there are exceptions. Perhaps a near-miss by a rogue planet in the past few million years could have started the moon spinning again. $\endgroup$ – IndigoFenix Oct 17 at 7:38
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Perhaps long ago a super advanced civilization decided that the planet didn't have enough land area and so built artificial continents.

Instead of bringing stone from stony asteroids and piling it up on the sea floor to build continents that were giant rock piles, for some reason the civilization imported a lot of metal from metallic asteroids and built gigantic metal pillars (presumably coated with anti corrosion substances) to support giant metal platforms that rose above sea level, and spread a thick layer of soil on top of the platforms to form artificial but somewhat natural looking islands and continents.

Or possibly the support pillars were made of advanced synthetic substances.

The story could be perhaps many millions of years later when the civilization that built the artificial continents is no longer involved with that planet, but there is a newer civilization the planet, either native or nonnative in origin.

And something happens to the support systems of the artificial continents for some reason and the support systems collapses. Parts of the platforms tilt a little but the width of the continental sized platforms is many, many times the total depths of the oceans so the continents mostly fall straight downwards. The continents displace water from beneath them until they get low enough that water starts to flow over the surfaces of the continents. Thus I expect that giant tsunamis will cross the oceans and strike the shores of any natural continents and islands of the planet, rushing far inland.

Of course the devastation caused by the giant tsunamis can be lessened by making only one small artificial island sink that way, or by making the artificial continents the only land on a planet with no natural continents or islands. Tsunamis crossing the deep open oceans should do little harm to lifeforms in the water, though they could stir up the sea bottom ooze greatly and kill all the creatures living on the sea bottom.

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Below Sea Level

A possible alternative is that the majority of your continent is actually below sea level, and you have something like a mountain range or various tall terrain features which actively prevent the sea from rising up and flooding everything (Think of the interior of Australia). Due to some extreme circumstances this natural barrier was destroyed (like a sink hole, or maybe a nuke or a huge earthquake) and this allows the ocean to pour in very quickly flooding the entire continent.

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Define "rapid", but in the terms you're talking about no, there isn't.

All the examples we have of submerged continents required millions of years to subside, and even then most of them still have portions above water. There isn't a realistic geological process that could cause such rapid subsidence over the sizes of land you'd be wanting that also didn't involve something that would cause at least an extinction level event.

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End of ice age

A lot of "land" on Earth is actually ice and snow coming all the way from the seafloor. For example, during some ice ages you could go from nowadays Russia to nowadays Alaska by foot. The ice bridge was also probably more than a mile above nowadays sea level.

Also check the answers for What would an Antarctica without ice look like compared to other continents?

The ancient people of the lost continent might have built their civilization on top of ice. When the ice melted, all their buildings and artifacts sank to the bottom of the ocean. Archeologists just think stuff was built on what is now the sea floor because they don't allow excavations and haven't checked the foundations.

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