If Japan had a population boom and had to expand their land area, on a very large scale, by draining the Sea of Japan, what is the most effective way of doing so in the shortest amount of time?

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    $\begingroup$ Step 1: Declare war on Russia and win. (Vladivostok is the main Russian port to the Pacific Ocean, and a large base for Russian Pacific Fleet.) (And anyway, Russia and the Koreas would probably want their share of the newly drained Sea of Japan. Japan needs to talk with them; and the relationships between Japan and Russia and the Koreas are, ahem, complicated.) Step 2: If you have defeated Russia, why bother draining the Sea of Japan when you have all Siberia and the Russian Far East? $\endgroup$
    – AlexP
    Dec 8, 2020 at 12:33
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    $\begingroup$ Apart from the geopolitical issues mentioned above, the Sea of Japan is a very fertile fishing ground. Destroying a major source of both food and exported goods, in the middle of a population boom, would not be a smart move. $\endgroup$
    – F1Krazy
    Dec 8, 2020 at 12:39
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    $\begingroup$ @Erik Funny! But keep in mind that "unpractical" isn't in Worldbuilding's vocabulary unless specifically asked for by the OP. Who cares if it's impractical? If it's possible, it's a valid (and frankly interesting) answer. (Heck, every James Bond movie in existence would never have been made if "practical" was a requirement of that world.) $\endgroup$
    – JBH
    Dec 8, 2020 at 13:11
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    $\begingroup$ Hello Celine! Welcome to our obviously happy family of Worldbuilding fanatics. Please ignore their good-hearted efforts to remind you that real life is important - everywhere else but here. I like this question and hope respondents will be magnanimously imaginative rather than boorishly bound by real life. Thanks for joining us! $\endgroup$
    – JBH
    Dec 8, 2020 at 13:15
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    $\begingroup$ It appears that the Sea of Japan is about 2.7% of the total global ocean surface area. Assuming an average depth of 1700m, displacing this much sea water would cause a global 45m sea level rise. I think this would be cause for concern for more than just Russia and Koreas. $\endgroup$ Dec 8, 2020 at 20:55

8 Answers 8


Draining a sea is not an easy job. The Dutch know something about it. And here you are not talking about a shallow sea, but of something with an average depth of 1300 m. Moreover you will have additional, non technical issues.

This is the Sea of Japan Sea of Japan

As you can see it separates Japan from Russia, North and South Korea. If you have ever been in an apartment block meeting where one person proposes changing the color of the shades and nobody else agree, you know it is a very long and complicated discussion.

This is the type of discussion you are going to start here, too, because you will be threatening the economic interests of your neighbors, who have more than just dog poo on your lawn to retaliate.

Moreover, the Sea of Japan is very important for fishery, so draining it is going to upset also a good part of the Japanese fishery industry.

If you really want to extend dryland, you might better look on the other side, on the Pacific coast, you might want to join Hokkaido to Honshu or Kyushu to Honshu, so that you won't be messing up with your neighbors but only with your internal politics. If you want to go big just fill up the Seto inland sea between Shikoku and Honshu.

Start by filling up the sea with excavated materials, then dry the waters when they are shallow enough, if you don't want to bother with reaching above the sea level.

This is something the Japanese have already done when building KIX airport and Odaiba in Tokyo.

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    $\begingroup$ Just to give an idea of the scale: The dutch had water of 2-9m deep to work with for their land reclamation projects. The biggest dams had to fill in those 8-9m and the reclaimed parts were 2-4m deep. Also, no real earthquakes or tsunamis. $\endgroup$
    – Cyrus
    Dec 8, 2020 at 14:27
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    $\begingroup$ We never filled our sea with excavated materials though (with a few exceptions). Usually we just build dams and then pump out the water in the middle. Compared to extinguishing the Sun this is easy even if the water is 1 km deep :) $\endgroup$
    – gerrit
    Dec 9, 2020 at 17:18
  • $\begingroup$ The straits between Honshu, Hokkaido, Kyushu, Shikoku, and Awaji (twice) as well as Awaji and Shikoku are vital for the Japanese cargo delivery and fishing industry - the numerous narrow straits are lined with the international harbors just as well as fishing harbors, and the shallow seas between Shikoku and Honshu are also prime fishing grounds as well as giving direct sea access to the south-eastern coast of the most industrial areas of Honshu. I'd propose to expand Okinawa instead $\endgroup$
    – Trish
    Dec 11, 2020 at 8:39
  • $\begingroup$ The airpoirs the japanese built are btw. built on concrete that was mixed with the ashes of their flammable waste. They are literally built on trash $\endgroup$
    – Trish
    Dec 11, 2020 at 8:41

Floating city.

There is a lot of it, but the water is fine. Build on top of it.


oceanix city enter image description here

Modular cities can be expanded as the population increases (a population boom in Japan is itself a pretty interesting concept!). They can be rearranged as "land' used change. Damaged modules can be floated out and replaced. Earthquake proof. Tsunami proof!

The water is fine underneath. In fact the fishing will probably be pretty good - fish will like these floating shades and will gather around underneath.

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    $\begingroup$ Great alternative idea! $\endgroup$
    – JBH
    Dec 8, 2020 at 16:05
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    $\begingroup$ Don't want to travel to the municipal library because it's too far away? Don't worry! Visit our website to figure out when the municipal library visits you! (I love this idea) $\endgroup$ Dec 9, 2020 at 10:15
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    $\begingroup$ It's an interesting idea, but good luck with typhoons, as they were a major factor in why Japan was never conquered. (Well, at least until WWII.) Many fleets, even large ones, have been sunk by typhoons, and as much tech as we have, we still haven't completely figured out how to deal with high winds on water. Something like this works best in a cove or other area protected from big waves and high winds. $\endgroup$ Dec 9, 2020 at 17:13
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    $\begingroup$ @Hoki - the problems caused by a tsunami happen when it hits land. Out at sea it is a giant moving hump of water. Floating stuff just rides up and over it. $\endgroup$
    – Willk
    Dec 10, 2020 at 17:31
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    $\begingroup$ @Hoki you can tie them to the ocean floor and to each other while leaving enough slack for tsunamis to fit under - which will, conveniently enough, recenter all of your floats ... and generate wave energy $\endgroup$ Dec 11, 2020 at 15:57

Behold! The Engineering Caisson!

I'll let you worry about the inconvenient details like the depth of the ocean, the amount of material you'd need to bring in, keeping surprised neighbors like Korea and China happy, and the usefulness of edible fish. You want to drain that sea! Let's do it!

In geotechnical engineering, a caisson is a watertight retaining structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. Caissons are constructed in such a way that the water can be pumped out, keeping the work environment dry. When piers are being built using an open caisson, and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.

(Image courtesy Weebly.com.)
enter image description here

In its simplest form, an engineering caisson is a big, long pipe that's driven down to, preferably, bedrock and the water pumped out. That lets you start constructing things inside it. You'd likely need to start with the pipe to create the connection from land to the sea. After that you want a "U" shaped caisson that connects to the last section of sea wall. This lets you extend the sea wall until you reach either the other side. Once both sea walls are built, you hook up some, well, impressive pumps and start draining the sea until you're to the point that you can start back filling it.

As Ash pointed out, there is an easier way

Now that you see how it can be drained, you can probably see the value of Ash's answer, which I up voted. You're going to back fill the space anyway, so why take the time to drain it in the first place?

The funny thing is, the world is already creating artificial coastlines by displacement. New land for buildings has been created in New York, Japan (curiously enough!), and at the top of the list is Dubai. Dubai made some of the most interesting new coastlines through displacement today.

So, displacement is easier and cheaper than draining the sea. But it could be drained.

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    $\begingroup$ I think you dramatically underestimate the scale of the challenge. The Korea Strait is 200 km wide and 227m deep in some points. You could probably make caissons, transport them and set them into place by letting them flood, but they'd collapse as soon as the water starts being drained. The dam needs to be as strong as the 3 Gorges dam to withstand the pressure of 100m of water, more to withstand tides, typhoons and earthquakes. The electricity requirement to keep the former sea dry is probably more than the world's current output, still +1 for a good image to be used in a story. $\endgroup$
    – Cyrus
    Dec 8, 2020 at 18:34
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    $\begingroup$ @Cyrus Cheers, but if you think about it, your first sentence is very much why I posted my meta discussion. Should we care that the problem is large, difficult, or would take the entire planet to even hope to complete? That's the OP's problem. If you think about it, the response could be to create a story that includes mega-machines that we can't even build today. Limitations... meh. James Bond saving the world from a single explosion in Silicon Valley that would cast CA into the sea... now that's worldbuilding. $\endgroup$
    – JBH
    Dec 8, 2020 at 18:46
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    $\begingroup$ If it is an engineering question, yes you should care. If it is a backdrop for a story-driven world, no problem. $\endgroup$
    – Cyrus
    Dec 8, 2020 at 18:54
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    $\begingroup$ No, @Cyrus... that's the point of my post. The site is limiting itself to "only what we can do now" and doesn't care that the OP may not care about those limits. It's certainly worth asking in a comment, but to exclude this answer just because it's unlikely we can do it today? Since when has that ever stopped a popular story from being written. Naw... don't let yourself be bound by too many facts. This is the site for flights of fancy. $\endgroup$
    – JBH
    Dec 8, 2020 at 18:56
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    $\begingroup$ "You want to drain that sea! Let's do it!" Now that's the kind of attitude we need in answers to this question. $\endgroup$ Dec 8, 2020 at 21:33

Don't drain it - displace it.

Draining it will require sea walls to be built. Those sea walls would be susceptible to flooding during storms, and this region gets strong typhoons with insane storm surges, and with sea levels rising due to climate change building below sea level is not wise.

So - raise the land up to above sea level using something your growing population makes as a waste product.

  1. Gather all your countries garbage in one space. Maybe buy other countries garbage too.
  2. Compact the garbage into solid cubes. 1 cubic meter of compacted municipal garbage weighs about 1.2 tonnes (481kg * 2.5 compression ratio)- it will sink in water.
  3. Wrap the garbage in plastic so it doesn't leak.
  4. Attach eyeloops to garbage cubes, and chain them together.
  5. Dump into Sea Of Japan.
  6. Repeat until the Sea of Japan has been replaced by a sea of garbage cubes just below the water line.
  7. Insert concrete / rebar poles for future buildings foundations.
  8. Cover with rocks of decreasing size.
  9. Cover with concrete if applicable.
  10. Build buildings.

Japan has already built an airport using a technique similar to this. Japan also has many mountains that can be dismantled for raw material should insufficient garbage be available.

This approach also has the advantage that you can do it piece by peice. You can take it in increments of 100sqm if you need to, whereas draining is all or nothing.

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    $\begingroup$ That'd be about 1.7 million cubic km of waste (volume of the Sea of Japan), that is 1.7 quadrillion (10^15) cubic m, that is 2 quadrillion tons of waste. Industrial waste aside humanity produces about 2 billion tons waste per year. So it would take only 500,000 years to achieve that. That I call long term planning. Please factor in plate tectonics. $\endgroup$
    – Erik
    Dec 8, 2020 at 13:08
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    $\begingroup$ I have to agree with @Erik. Besides making a base of polluted ground, it seems unfeasible as you need to basically excavate more ground than the whole of Japan currently has. $\endgroup$
    – Trioxidane
    Dec 8, 2020 at 14:55
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    $\begingroup$ Here in the U.S. we make new ground out of polluted ground all the time. We've built everything from golf courses to urban residential expansions over old land fills. The quantity might be an issue (You'd need to move a good chunk of Australia to do this... they won't miss the bush, right?) but it's a completely feasible idea! $\endgroup$
    – JBH
    Dec 8, 2020 at 16:07
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    $\begingroup$ About #3. Mother Nature is in process with developing critters who feast on plastic. Depending on plastic to prevent leakage may not be advisable. $\endgroup$ Dec 8, 2020 at 21:22
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    $\begingroup$ @ChrisH, actually, mountains have historically been a good place to grow crops. The slopes give a bit more area than flat ground, and also give good natural drainage for rice paddies. So good that they have to dam up the paddies to make it work. But taking off the tops down to a useful level was also my thought. We just need to replace the tops with a few feet of soil, so people aren't just looking at a granite slab, or whatever those mountains are made of. $\endgroup$ Dec 9, 2020 at 17:19

We at MegaGeoCorp stand ready to offer you the sovereign solution to all your ocean draining problems!

Now, it's true that draining a whole sea is a bit of a technical challenge, but really, the challenge is but one of scale. A little siphon pump can draw up water from a rain barrel very handily, for example. For this job, we just use bigger pumps!

But that's just one part of our Two Part Plan. You see, MegaGeoCorp has been working tirelessly with the Australian government on reclaiming their ancient Inland Sea. As you can see from our project specs, we're experts in the excavation, hauling, and recycling of vast quantities of rubble. Our Megadiggers and Megatrains and Megamovers have been busy eating through the Outback, creating a new sea basin and building up long eroded mountains.

However, we discovered a wee problem in our calculations. For, you see, it turns out our summer planetary engineering intern severely underestimated the amount of rubble we'd be shifting. This means we've got boulders and bongoliths, rocks and crags, just piling up in our worksites!

Therefore, MegaGeoCorp's offer is simple: as our great work in Australia winds down, we'll use all our Megaships to transport all that extra rock the Australians don't want and tip it over at various strategic locations around Japan. This will form the Great (Underwater) Wall of Japan, which will stretch from southeastern Korea down to southern Japan; another wall segment will connect the main islands of Japan; a third wall segment will stretch from northern Japan on over to eastern Russia. Easy peasy! We estimate that we've dug up enough Australia to make a wall ten miles wide, easily!

With all that extra rubble forming huge dykes, it will be an easy matter for our patented Megapumps to remove as much or as little water as you need.

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    $\begingroup$ Congrats for an answer which isn't a frame challenge. $\endgroup$
    – Nepene Nep
    Dec 9, 2020 at 15:30
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    $\begingroup$ With some luck the water level may drop a bit even before you do any pumping (see Atlantropa). $\endgroup$
    – gerrit
    Dec 9, 2020 at 17:22
  • $\begingroup$ With some further luck, you can make the mexicans pay for it! $\endgroup$
    – Dschoni
    Dec 10, 2020 at 10:35

As you are looking into draining, it makes sense to enclose the waterbody first which is a simple engineering problem [citation needed].

Once that's done, we are left with some options. Let's have a look.


The amount of water in the sea of japan can be calculated by taking it's surface times it's mean depth. According to Wikipedia, the surface is about 978 000 km² with a mean depth of 1 752 m. This gives us about 1 729 224 km³ which equals 1.8e18 litres of water. The world's most powerfull pump, that is actually used to keep the netherlands dry, can get rid of 60 000 litres per second. This pump would therefore only need ~914 thousand years.

But why stop there?

The south-east wall to limit the ocean would be around 124 km long if you build over shimayama island. enter image description here

As the pump has a head size of about 5m, let's assume we could put one of these pumps in the whole wall every ten meters. That would reduce the time it takes to pump out all of the water to about 74 years! It would use about 50 GW during this time, which would make ~42 more nuclear power reactors necessary.

[\begin edit 11.12.2020]


So you decided to build 42 nuclear power plants but unfortunately the producer of the pump went bancrupt meanwhile.

Fear not!

You can use your excess power to turn all the water into hydrogen and oxygen (there might be serious side effects to global climate, but we are here to drain an ocean, so why bother?). To fully turn one litre of water into oxygen and hydrogen, we need approximately 13MJ. So the whole sea of Japan can be turned to cloudy madness with only about 2.3e26J. Having nifty 50 GW = 5e9 J/s of nuclear power at your hands this will "drain the sea" in about 4.6e14 seconds or about 14 Million years. You would have a lot of hydrogen left, that could be useful to turn into power (or water) again... [\end edit 11.12.2020]


You could heat the enclosed basin to boil the water and evaporate it. Getting the steam from condensating and raining back into the ocean is another obviously easy engineering problem [citation needed]. Back during world war II, the nazis thought of building an enourmous mirror in space with the exact purpose of using it as a weapon incinerating cities and evaporating oceans. While this sounds like a viable option, there is a way easier method of heating a body of water rapidly to above boiling point.

Throw nuclear bombs at it.

Sea water has an average temperature of 17°C with the sea of Japan being a little below that (assuming 10 for now). Water has a specific heat of 4.19 kJ/kg°C so heating the whole basin to 100° would be 1.8e18 * (100-10) * 4.19 kJ/kg°C = 6.8e20 kJ.Furthermore, the latent heat of water at 100°C is 2.26 MJ/kg. So add another ~4e21 kJ to the bill.

The nuclear bomb with the highest energy yield ever built is the Tsar Bomba with an energy output of 210 000 TJ = 0.2e15 kJ. You would need to have 20 000 000 of this bombs, but once you explode all of them at the same time, the basin should be empty in matter of seconds.

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    $\begingroup$ I wonder what exploding 20 million 50 Megaton nuclear bombs would do to the rotation and tilt of the earth. $\endgroup$
    – cjnash
    Dec 10, 2020 at 15:01
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    $\begingroup$ I just wanted to ask "why drain when you can boil it?", nice! $\endgroup$ Dec 10, 2020 at 16:38
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    $\begingroup$ @Dschoni thank you for pointing that out. I feel the question is valid, but I strongly disagree with the nuclear answer proposed. The answer does fail to understand that Japan would not even allow any other country to bring nukes into their territory - and in case of the USA: there's a treaty that bans the USA from even transporting them through Japanese waters. I even wrote more in the meta on that. $\endgroup$
    – Trish
    Dec 11, 2020 at 11:37
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    $\begingroup$ @Trish I think you are taking this answer too seriously. We are discussing building a nuclear arsenal roughly ten thousand times as large as the total of all the nukes built by humankind ever, possibly requiring more uranium/plutonium than exists in the solar system, then using it to instantly boil an ocean, in an incalculable waste of resources which will probably also destroy all life on earth. Convincing the world's most nuke-averse country to participate in this scheme is the least improbable part. $\endgroup$ Dec 11, 2020 at 20:43
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    $\begingroup$ +1: the list of ways to make water go somewhere else, with absolutely no other considerations taken except for how much electrical energy will be needed. $\endgroup$
    – Mazura
    Dec 12, 2020 at 2:59

Welcome to world building.

It depends how short the time is and how much the land needs to be extended by. The premise of draining the sea of Japan is not remotely realistic at all and even assuming all political issues were to vanish and the entire world became dedicated to this task (for some bizarre hand waved reason) it would probably not be practical on anything other than a time scale of centuries at the very least. The Korean straight is around 120 miles wide and averages at 300 feet deep and the sea of Japan is up to 2 miles deep.

Assuming that land must be reclaimed, the best starting point would be to extend out from the shallow coastal waters. Here a relatively large amount of land might be reclaimed for a comparatively small cost (compared to draining the sea of Japan that is).


The displacement theory gave me another idea: Use materials from volcanoes to displace the water. Because just moving the amount of material needed to fill the sea seems out of reach for the foreseeable future, just considering the energy needed to do it. But if we could incentivize enough lava to spill out of the ground, earth could do it for us.

Sadly, the "ring of fire" seems to be more on the eastern side of Japan, less so on the west side. But there could be some "close enough" areas in the sea of japan.

Even though Japan does currently not have nuclear weapons, they might change their doctrine to be able to acquire them for "geo-engineering" purposes without having rockets or planes to use them on other countries, then detonate them on the sea floor close to some fault lines.

The underwater detonation would probably (I'm not an expert on this) cause tsunamis on all the shores around that area, so there's the problem with the neighours who would probably see this as an attack. There could be more tsunamis following if the other fault lines in the area get disturbed by the underground activity.


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