There's an island. Maybe it is a continent, maybe not. I am not sure and that is why I need your help.

If the size of my world is equal to Earth's, what landmass size would it take to drop the sea level by the aforementioned amount if it were to, say, lift up into outer space?

I know it sounds silly, and is unprecedented, but a huge part of my mythos revolves around the discovery of new lands as a result of receding tides and more extensive coastlines.

I hired a cartographer who pointed out that by doing this, I would weaken the integrity of oceanic crust, kind of like thinning the skin of a balloon, potentially leading to an extinction-level event later down the road. Would there not be potential for a new island or chain of islands to lift up as a result of the magma expulsions? Would this heat the temperature of the world, poison the oceans, darken the sky with contaminants, etc.? I would appreciate input on anything.

I don't need any other explanations for why all of these events could be happen.

  • $\begingroup$ Hi, welcome to Worldbuilding.SE! Please take the time to read through our tour if you haven’t yet and visit our help center if you need more information. I would encourage you to visit the Sandbox on Worldbuilding Meta if you are unsure if a question is suitable for our site. I also encourage you to visit our list of worldbuilding resources for inspiration and help with general questions: worldbuilding.stackexchange.com/questions/143606/… $\endgroup$ Apr 26, 2019 at 18:07
  • $\begingroup$ Can you clarify your question some? Are you asking how much material you would need to remove from the earth in order to drop the sea level by 10 meters? If so, your answer is simply “you need to remove ~5 meters of material from the sea floor” (~5 as the earth is spherical, despite what those flat earthers say, so we are using a radius, the total diameter would be 10 meters meaning the oceans have lowered by 10 meters) $\endgroup$ Apr 26, 2019 at 18:14
  • $\begingroup$ @LiamMorris That's the guts of an answer - maybe you could convert it to one. :-) $\endgroup$ Apr 26, 2019 at 18:44
  • 1
    $\begingroup$ What's wrong with the usual mechanism of altering the sea level by changing the amount of water trapped in the polar ice caps? In the Eocene the sea level was some 100 meters higher than now, while during the Last Glacial Maximum it was some 100 meters lower than now. Note that a puny 10 meters regression won't buy you that much land. $\endgroup$
    – AlexP
    Apr 26, 2019 at 19:56
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    $\begingroup$ @LiamMorris Both the sea level and the ocean floor level are radii. So either both are 10 meters or neither. There is no converting to diameters by dividing by two. $\endgroup$
    – Brythan
    Apr 26, 2019 at 21:20

2 Answers 2


Let's assume a landmass disappears from the face of Earth, leaving a hole with the depth of average ocean's depth - 3.7 km. Let's also ignore any continental shelf that our disappearing land has and assume that it just leaves a clean hole 3.7 km deep. The surface area of Earth's oceans is 361,900,000 sq. km. Let's ignore the shallows and assume that the amount of water to be drawn out equals to 10 m times the surface. this gives us 3,619,000 cubic kilometers. This gives us the area of 978,100 sq. km. This is bigger than the size of New Guinea (785,753 sq. km), but much smaller than Greenland (2,175,600 sq. km).


I'm not sure I understand your question correctly. You want new land to emerge from receding water levels. And you want there to be a myth that the water has fallen because an island has disappeared. Let's answer these separately:

1. New land emerges from sinking ocean levels

Historically, sea level has fluctuated by about 400 meters:

Graph of sea level fluctuations over the last 542 million years

Causes were global temperature changes and the resulting different amounts of water bound into glaciers as well as changing ocean volume due to continental shift. Some of this is explained on Wikipedia: https://en.wikipedia.org/wiki/Eustatic_sea_level

So all you need is an ice age. Or a landmass sinking due to plate tectonics

2. Myth of island disappearing

Myths don't usually recount historical facts, but are attempts by people to explain things they don't understand using the world view they have.

So a myth of an island disappearing does not have to mean that an island has actually disappeared. It could be an attempt at explaining sea level changes following an ice age.

You can have whichever myth you want, independent of the true events.


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