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So I've been trying to create a world and I've been planning on the continents being drowned out almost completely in water from a terrible tsunami, storm, earthquake or an act of one the gods or something, maybe a mixture even.

The land before would have very deep valleys and would be close to sea level to begin with, so maybe some of the land would still be above water, but most of the continent would be underwater, and wipe out a good portion of its citizens.

More specifically I suppose,

  • What would be a more plausible reason (if the ones listed aren't too good) to lose a continent in a way it wipes out most of its population?
  • Would there be survivors if they were able to board ships in time to sail over to another continent in time?

I've read up on Mu and Lemuria as presumed lost continents but it doesn't exactly answer my questions.

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    $\begingroup$ A tsunami simply moves water, and is not significantly different from other waves in water except in cause and magnitude. Unless there is something to capture and prevent the water from receeding, after the wave itself recedes things will go back more or less to what they were before (sans any objects on land or near shore that were displaced by the onslaught of water). $\endgroup$ – a CVn Jan 23 '15 at 8:39
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    $\begingroup$ "very deep valleys" and "close to sea level" - presumably the land at the top of those valleys isn't that close to sea level... $\endgroup$ – AakashM Jan 23 '15 at 10:25
  • $\begingroup$ @AakashM: Not necessarily. The valleys could go below sea level in theory. It'd be unusual but when world building that's quite usual. :) $\endgroup$ – Chris Jan 23 '15 at 14:16
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    $\begingroup$ @Chris not river valleys then :) $\endgroup$ – AakashM Jan 23 '15 at 15:09
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    $\begingroup$ In fact for terrible floods causing devastation I highly recommend some reading about the Zanclean flood which ended the salinity crisis I linked to. en.wikipedia.org/wiki/Time_%28xkcd%29 talks about the XKCD comic that first introduced me to the concept and is about people living at the time of an event such as the Zanclean flood. Its not flooding a continent but it is a large area of land getting submerged. And very interesting on its own. $\endgroup$ – Chris Jan 23 '15 at 15:56
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There is Doggerland, which connected Great Britain and Mainland Europe. It disappeared during 6,500 to 6,200 BC due to rising sea levels.

Quote from the Wikipedia article:

A recent hypothesis is that much of the remaining coastal land, already much reduced in size from the original land area, was flooded by a megatsunami around 6200 BC (approximately 8200 BP) caused by a submarine landslide off the coast of Norway known as the Storegga Slide.

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    $\begingroup$ Welcome to Worldbuilding :-) $\endgroup$ – Mourdos Jan 23 '15 at 8:58
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    $\begingroup$ "Mainland Europa" Considering that Europa is a moon of Jupiter, I think you mean Mainland Europe. $\endgroup$ – Joshua Jan 23 '15 at 19:31
  • $\begingroup$ @Joshua Depends on which country you're from.. ;) Kidding aside, you're right, luckily, someone edited it for me.. $\endgroup$ – Tobias Wärre Jan 28 '15 at 6:48
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One possible cause would be a natural dam that breaks, like in the Black Sea deluge hypothesis.

In that case, it is claimed that the Mediteranean Sea opened the Bosphorus Strait and flooded in the Black Sea, which at the time was one hundred meters below sea level.

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The end of an Ice Age has done it here on Earth. The ice contains enough water to alter the sea level significantly and when the warming is fast it is normal for some of the melt water to get trapped by the still unmelted ice, so the sea level change may come in pulses. In short, a collapse of a large ice sheet releasing lots of melt water sounds a like a feasible reason for the sea level to change relatively fast.

To flood a continent you need a rise in the global sea level, the seas are too wide for this to happen very fast. Local flooding that follows, such as the one when the Strait of Hormuz was created, can be quite fast, but even then people still survived. Ships are built to float so they are rather good at it unless something breaks them.

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  • $\begingroup$ Note that only ice supported by land will impact sea levels in any significant way when it melts. Due to ice's lower density than water (which also results in its buoyancy), melted sea ice will basically fill the hole in the water that the ice creates. If a continental ice sheet is particularly large, it can weigh down the land, causing the latter to rise up as the former melts, a process called "post-glacial rebound". $\endgroup$ – outis Jan 24 '15 at 10:45
  • $\begingroup$ Note that there is even some archeological research done on now-submerged areas of continental shelves, which were above sea level during the last Ice Age. One theory for why there is little evidence of the human migration into the Americas is that they travelled along the now-submerged coastline. $\endgroup$ – jamesqf Jan 24 '15 at 23:43
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Check out Google Earth east of Florida tucked in above Cuba. Even from way out you can see something different about that area. Checking to see if that's an artifact of the patchwork, I found that it's a land mass lower than sea level, as opposed to miles-deep ocean. The tallest peaks form the Bahamas.

I think that's what a lost continent would look like. The question you asking is could it happen suddenly? Maybe the land was quite low lying due to generations of sea-level rise, or had a basin surrounded by mountains near the perimeter. A tsunami could cause widespread damage. In the latter case, breaking through the ridge would cause a new inland sea to fill up...like the Mediterranean did, or the Black Sea which occurred while humans were around.

The biggest floods, in North America, was caused by ice dams failing during periods of glaciation. In this case it was scrubbed clean but did not remain underwater.

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3,600 years ago, in the Santorini archipelago, a huge volcanic eruption happened. This led to a tsunami so big that it supposedly wiped out the whole Minoan civilization, which potentially gave rumours to the lost city of Atlantis.

reference

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What if the species was environmentally conscious enough and technologically advanced enough to house the bulk of its operations underground? It has been done before:

http://www.history.com/news/vast-underground-city-found-in-turkey-may-be-one-of-the-worlds-largest/turkish-underground-city

Imagine the absolutely insane network of tunnels required to support 500,000,000 mouths. The earth supplies an inexhaustible and unassailable supply of geothermal energy. You could even have a transient cataclysm that drove them underground, effectively--increased radiation from an extrasolar event, comet or asteroid--jumpstarting their civilization. We could and probably would do it given a few thousand years warning (predictable extrasolar event, such as a nova or supernova, fusing stars, magnetar, quasar, whatever).

You can eradicate their surface presence in prehistory via multiple avenues. You really just need a compelling reason to break their connection to the surface. A strong flood, tsunami, asteroid/comet impact, even megalomania.

They could be beneath us right now, or they could all be dead, their chambers hermetically sealed and filled with advanced technology that harvested ambient energy with startling efficiency--acoustic engines and whatnot.

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What about the same plan in the 20s to create a 10X larger dam than the Three Gorges Dam. There was a plan called Atlantropa (see wiki). The Dam was to be situated at the mouth of the Mediterranean and was never built as per engineering and other limitations of the era--as well as the risks that a breakdown of the Dam would have created massive floods across the Mediterranean. The project was planned to create energy and to an extent agricultural land across the coast lines in the Mediterranean. Maybe you could devise an ancient civilization having constructed such a project and having a breaking of the Dam. Although the Dam was planned to have reduced the level of the sea by 200 meters. So you would need a shallower landscape.

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