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I am envisioning a scenario in which, probably for reasons of persecution or overpopulation, a people would begin to construct islands in the sea. Each island would be fairly close to the others and the society would want to be able to support agriculture on the islands. I had two questions about this idea:

  1. Would the only way in which an island could come about in the ocean be by volcanic (and non-magical) means (eg like Surtsey) - or would it be possible for them to construct non-volcanic islands simply by transporting the necessary materials to a favourable location in the sea and beginning construction? I know that China is currently constructing artificial islands by non-volcanic means, but the society's technology would be roughly similar to that of Europe during the Age of Exploration, so this may affect the techniques that they could use. Would they be limited to flat islands or could they construct rock features such as mountains etc?

  2. If this was possible what materials would they need to use for the various layers supporting the island? - going on the premise that each island would be roughly the size of a small city or town.

Thanks for any help

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    $\begingroup$ Missing big example: Venice. built on wooden pylons $\endgroup$ Feb 16 at 19:01
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    $\begingroup$ Age of Exploration technology is sufficient to build unstable islands. Pick up rocks, dump them in the ocean and pour sand on them. The problem is you don't have a vibrator to prevent liquefaction. The Palm Island, Dubai UAE - Megastructure Development, YouTube $\endgroup$
    – Mazura
    Feb 17 at 1:59
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    $\begingroup$ As a new user, please keep in mind that asking more than one question (no matter how related) is a reason to close a question (see Needs More Focus). For the future, it's better to ask a series of questions than trying to post them all in one. In fact, you'll often find that the answers to the first question will change the following questions. $\endgroup$
    – JBH
    Feb 17 at 7:15
  • $\begingroup$ Probably easier to hit your neighbour and make him move to somewhere else while you keep his land - also called war. Or to just go and live somewhere else: en.wikipedia.org/wiki/Migration_Period $\endgroup$ Feb 17 at 14:24
  • $\begingroup$ Another example -- much of harbor-side Tokyo is reclaimed land, filled in starting from 1592. From en.wikipedia.org/wiki/Tokyo#1945%E2%80%93present -- "Land reclamation projects in Tokyo have also been going on for centuries. The most prominent is the Odaiba area, now a major shopping and entertainment center." See also Venice, as mentioned above, and a big chunk of the Netherlands, particularly the polders: en.wikipedia.org/wiki/Polder $\endgroup$ Feb 18 at 23:59

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Move the ocean off of the top.

Piling stuff in the ocean to make an island has a long track record. The Chinese are doing that now on a 21s century scale.

But what about just moving off the ocean and keeping it off. The Dutch have been doing that for millenia.

https://www.thoughtco.com/polders-and-dikes-of-the-netherlands-1435535

Pushing Back the North Sea For the next few centuries, the Dutch worked to slowly push back the water of the Zuiderzee, building dikes and creating polders (the term used to describe any piece of land reclaimed from water). Once dikes were built, canals and pumps were used to drain the land and to keep it dry.

From the 1200s, windmills were used to pump excess water off the fertile soil, and windmills became an icon of the country.

I propose your people make an "island" that is below sea level. Technically they make a big island with a big hole in the middle. The fertile farmland on the reclaimed seabed is surrounded by dikes and polders and full of windmills but is not a straight ripoff from the Dutch because it is way out at sea. Incoming ships tether on the outside of the dikes and people go down ladders on the inside.

Cool and fantastic for a built world and you could have it with older technology. Although people are doing it with new tech. The best image I found was from Singapore!

singapore reclaims land

https://www.hydro-international.com/content/news/singapore-pilots-dutch-land-reclamation-approach

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    $\begingroup$ Also consider the Dutch - the linked source says that a dike break in the 1200s killed 50,000 people. And then they built it all back. The Dutch got into their share of wars. Not sure how they kept people like the English from blowing up their dikes but maybe it is harder than it seems. $\endgroup$
    – Willk
    Feb 16 at 19:06
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    $\begingroup$ @Willk Actually, it was more often the case for the Dutch to intentionally poke through some dikes, flood part of the country to a specific depth that's too deep for infantry to wade through but too shallow for boats, and use it to stop an invading force. It worked in several conflicts, including the so-called Disaster Year 1672 (war against England and France combined); only time it failed was WW2 when the Germans had planes. $\endgroup$
    – KeizerHarm
    Feb 17 at 10:34
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    $\begingroup$ Also note that behind the dike is civilian cities and farms, not military infrastructure. Destroying one doesn't hurt an enemy's military capability immediately. Plus, totally flooding a city is the equivalent of carpet-bombing it. You don't do that unless you have a total war; most medieval warlords wanted to conquer places roughly intact, to own it themselves. $\endgroup$
    – KeizerHarm
    Feb 17 at 10:38
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    $\begingroup$ Do note that the land reclaimed from the sea isn't as 'fertile' as you might think. It is definitely fertile, but salty. Iirc it is the reason for many tulips, as they are resistant against salty soil. $\endgroup$
    – Trioxidane
    Feb 17 at 18:19
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    $\begingroup$ @LorenPechtel - thats what windmills do best $\endgroup$
    – Willk
    Feb 19 at 5:03
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Man-made islands in Scotland and indeed very old - upwards of 5,000+ years old, so yes it is very possible. Citing from the second link:

Prehistoric tribesman rolled blocks weighing up to 25 kilos into the water to form short platforms you can still see today – though many are now covered by trees and other plantlife.

You probably would start with a man-made bridge (platform), and then just put a ton of them together in the sea to make an "island", before removing the stones used to make a platform. In other words, if you can make a land-bridge you can make an island, of some form. Perhaps the most notable example of a land bridge was Alexander's in 332 BC during the Siege of Tyre.

As far as what materials and/or structures are required, that really depends on numerous factors:

  • depth of the sea
  • mass required to be supported
  • current (force of the water on the various layers underneath)
  • waves / sea-turbulence (surface pressure)

For example, village-style stucco homes in a shallow, non-turbulent sea will require a lot less than multiple skyscrapers in the middle of the Antarctic. Seeing as though small towns could be built in the Scotland-style islands, all you need is a lot of very big rocks. Once you have that, pack in dirt and get moving. Again, though, the environmental factors will drastically vary the requirements.

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    $\begingroup$ Midden heaps could work too - I read somewhere that there were islands in the US formed from clam shells that the Native Americans discarded. What's nice about this is that the island protects itself - new clams glom on to the exterior of the pile, protecting it from erosion. $\endgroup$
    – codeMonkey
    Feb 17 at 16:33
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I'm fond of a quote from the movie, The Hunt for Red October:

"Can you launch an ICBM horizontally?"

"Sure, why would you want to?"

Could nations during the Age of Exploration build artificial islands? Sure! Though they'd likely not be able to build one large enough for your purposes before the Age of Exploration ended. Bring in enough boulders to create a continuous barrier around the perimeter of the island (complete with docks... of course!) and fill the center with dirt until the water's gone and the dirt dries out.

But what's the point? The existence of an artificial island without desalination (invented in 1964) would require some method of consistently accessing fresh water. Yes, you could build a giant lake in the middle of the island (now that perimeter's getting big...) to catch rainwater, and you can build the perimeter barrier high enough (maybe...) to keep wind-driven waves from surging over the barrier and ruining the fresh water....

But what's the point? All that effort when it's thousands of times easier, cheaper, and more efficient to simply move an inland mountain to an existing shore and extend an existing natural landmass — and you need to move the mountain anyway just to build your island. No matter how shallow the ocean where you build the island, you'll need at least one mountain worth of dirt to make an island big enough for practical cultivation.

And you still can't ignore that barrier! Coastlines exist for reasons that can't be ignored without consequence. Just look at the Dubai attempts to make artificial islands which, having lost access to regular maintenance, are slowly eroding back into the sea. And what really hurts is that your barrier is perhaps the most expensive part of island-building. You wouldn't want a bunch of small islands because every inch of perimeter comes with a very high cost.

So... why do you need artificial islands? Is it just a cool idea to have in your story/game/world, or do you have a purpose (like establishing strategic colonies for defense or expansion)? If we knew why you needed the islands, we might give you a better answer as to how the islands can come to pass.

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    $\begingroup$ The purpose for the islands would be either overpopulation or persecution, in both cases to escape the mainland, but admittedly I was sort of engineering the circumstances in order to include this feature. $\endgroup$
    – ermatveit
    Feb 17 at 9:07
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    $\begingroup$ @ermatveit Then I'd recommend that you not worry too much about how to make it "realistic." It's a cool idea and I'd hate for it to get bogged down in the impracticalities of "reality." $\endgroup$
    – JBH
    Feb 17 at 9:14
  • $\begingroup$ I think the islands in the penultimate paragraph are in Dubai, not Saudi Arabia. $\endgroup$
    – MSalters
    Feb 17 at 16:02
  • $\begingroup$ @MSalters Thank you for the catch! $\endgroup$
    – JBH
    Feb 17 at 17:52
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How about starting with something like a raft - it requires constant maintenance, so you keep adding new material, you plant sail-resistant trees (eg mangrove), whose roots bind the structure together, you add new material on top as it sinks deeper, and along the edges - and one day you realise the thing is touching the bottom in places. Shortly after, your now very big raft has become an island.

Perhaps not completely realistic, but with a combination of the right materials, plants, a culture of constant vigilance and clever application of general handwaving, you make it work.

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Have you considered the amount of material it would require to do this? For example, just to carry the material used in the construction of the Washington Monument would take somewhere around 300 shiploads from a typical carrack ( most common merchant ship of the time ). I would imagine that even a small island built in shallow waters would require 10s of thousands of such shipments.

The illegal Chinese islands are built largely by dredging up sand and dumping it on existing islands or coral reefs. But our 15th century island builders wouldn't have the technology for that ( and might not want to do it anyway given the erosion problem the Chinese are having), they'd be digging and quarrying material on shore and then hauling it to a port to load on ships.

To carry this off you would need to be wealthy and powerful enough to take control of nearly all of the shipping on the planet. You would also have to be wealthy and powerful enough to carry out a massive material gathering and transportation effort to get the material to your ships. And that wealth and power would have to be stable enough to let you do this for decades.

And when you're done, you're left with small islands that wouldn't have any natural source of fresh water. You'd be very limited in what crops you could grow because even the soil you're bringing in would be getting polluted by salt water spray and seepage.

There is theoretically possible and there is realistically possible. Theoretically, it's certainly possible that 15th century people could create islands, including ones with something akin to mountains. Realistically, there's really no way it would have happened.

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