Ideally using today's technology, or that of a few decades in the future, how might scientists engineer an island, floating in the sea1 fulfilling these criteria?

  • it is large enough for humans to live on it, and self-sufficient
  • it is predominantly plant-based, or similarly living, and so
    • can repair minor damage (e.g. loss of small quantities of buoyant material) automatically
    • can expand (to some extent) automatically
    • more major damage (e.g. loss of entire "boughs" (or equivalent)) is fixable, albeit perhaps with human aid
  • it would remain far away from land
  • (Bonus: it can be moved around the ocean, directed by its inhabitants)


It seems to me that a single organism might be impractical; however, would some sort of scaffold with kelp, or some similarly buoyant waterweed, growing from it be possible?

1. Questions have already been asked about islands floating in the sky; this is not one of them.

  • 1
    $\begingroup$ Just to be clear, this is floating on the ocean, yes? Not in the air? $\endgroup$
    – Dubukay
    Commented Jun 24, 2018 at 20:04
  • $\begingroup$ Welcome to Worldbuilding, Benedict Randall Shaw! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ Commented Jun 24, 2018 at 20:04
  • $\begingroup$ @Dubukay oops yes, sorry $\endgroup$ Commented Jun 24, 2018 at 21:31

4 Answers 4


The biggest problem you have is storms. Anything large enough to support even a tribes worth of humans is going to be too large to survive a storm. You simply can't build anything that large that strong wave action cannot rip apart. You're just bound by material strength to weight ratios here even hollow steel has limits. It is also impossible to steer something that size. Instead you go with many smaller floating objects enough for one per family. So basically organismal boats forming a flotilla.

there are a few ways to go about this:

  1. something like floating bamboo like pontoons might be a good choice, as grasses they can expand organically one pole at a time and excess can be used for building material. this can give the illusion of a larger floating forest, downside not easily steerable and will require a lot more effort on the human part. up side as storms rip them apart what is left still floats and can grow.

  2. Alternatively growing an actual ship from a single tree would be very potent. One piece of wood comprising the whole thing with branches and leaves instead of sails. Normal wood will not work for this, you need wood that grows in a patchwork pattern, almost "cellular" but on a macro scale as wood itself can't grow. Alternatively it can grow outward with a bark like covering, however your ships will then be largely solid then unless the humans carve out the inside, but they will have a hard time finding functional tools for this. either way letting the grow can get something equivalent to the largest seaworthy wooden ships ever built without much issue, you also end up with ships of every size, the smallest saplings might be slender canoes or rowboats. This adds a lot of versatility and lets people move around a lot easier. you will still need other types of things to get fuel from, maybe floating sapling farms.

You will need lots of other plants for cordage and such your people will have subsist of mostly fishing. You will also need some organic form of water purification. you can also combine both ideas for diversity.

keepin it far away from land will be up to humans steering it, there is no way to keep something that flats from eventually making landfall without steering it.

Keep in mind without mineral based technology your humans will be stuck with "stone age" technology (without the stone) there are modern hunter gathers who live like this all exclusively island dwelling were sand is the major constituent of the islands with no serviceable stone. Fire is an essential human tool so your "ships" will need to generate a lot a fuel.


There is a wealth of material on both naturally occurring and man-made islands. Some are actually already with human settlements on them, and self-sufficient.

Naturally Occurring Floating Vegetation Islands

Most naturally occurring vegetation islands are formed by reeds, sedge, cattails or bulrush extending outwards from a shoreline, then when the water level increases due to unseasonal rain they detach from lake-bed to form a floating island. Over time these may perish, but enough exists to form a bed for other plants to grow. If the water level is sustained, these floating islands can be quite permanent, with many examples in Europe, the Amazon and Mexico.

enter image description here

Artifically made Floating Islands

Have a read of the Uru people in Peru. For defensive reasons they created 120 floating islands of totora reeds, large enough to contain settlements from which they can live and fish from. Their population is about 2600.

Each island lasts about 30 years as the reeds do rot and float away, even if the layers on the surface are healthy, so there is a constant part of their livelihood to add more reeds over time.

enter image description here

In the Open Ocean

Many of the above examples are in lakes or fairly static bodies of water. In the ocean is a different story, you need a strong material to withstand stresses in the open ocean. There was a proposal during WWII to construct an aircraft carrier out of wood pulp and ice, pushed by two-dozen motors.

The project was called Habbakuk, and the idea was to have a 1200m by 180m, 12m thick, floating island to launch heavy bombers. With a range of 11000km, it was impervious to torpedoes which was a large advantage at the time.

It was cancelled once aircraft increased in range, and also once more traditional lighter escort aircraft carriers were produced in increasing numbers, however it is a good demonstration it could have been done - with some effort.

  • $\begingroup$ note those are on lakes or rivers, in the ocean they would be torn apart by the first storm, if not sooner by wave action. $\endgroup$
    – John
    Commented Jun 27, 2018 at 20:42

Balsa Trees (Ochroma)

Using balsa wood due to its large strength-to-weight ratio and other buoyant plant materials on top you can create a stable foundation much like a raft however you'll also need a keel to prevent your island from tipping over and a hull to act as a retaining wall for your floating island, that way you can plant more balsa trees, which could be used for replacing any damaged sections. In the end you're basically constructing a ship that is basically almost level with the water and you just end up in an endless battle of forces to stay buoyant.

Possible challenges

  • Salinity, maybe use mangrove systems beneath the foundation to allow salt water to somewhat enter creating basically a sort of mini freshwater table under your island.
    • Sinking depending on how much water is allowed to seep through.

Artist Interpretation

enter image description here


So, we're looking for a plant-based material that'd be used to construct a floating seaborne island large enough for humans to live on it, which could then be self-sufficient, can be repaired and expanded, and which would remain far from land? In which case, I nominate Southern Bull kelp, aka as Durvillea Antarctica. Also known as "Rimurapa" by the Maori, "Rimuroa" by the Moriori, and as "Chocayuyo" by the Quechua, Aymara and Mapuche.

This species of brown seaweed, found across the Southern Ocean all the way to the shorelines of Antarctic, doesn't have air bladders- the seas of the Roaring Forties, Furious Fifties and the Shrieking Sixties are far too rough, raging and turbulent for that. Instead, it floats due to a unique honeycomb structure within the alga's blades, which also gives it exceptional strength and buoyancy, and helps the kelp avoid being damaged by storms. D. antarctica has to resist forces equivalent to winds of over 1100 km/h.

This is thought to be responsible for the wide circumpolar distribution of this genus, as the kelp is able to float when its holdfast fails (usually as the result of worms and molluscs eating away at it, or of the rocks it's latched onto breaking away). It can colonise other coastlines in this manner, and has been shown to carry communities of invertebrates across vast ocean distances from one shore to another, with this 'rafting' with Durvillaea antarctica especially allowing a wide range of species to island-hop and to re-colonise sub-Antarctic shores scoured clean by ice during the last Ice Age.

The southern Māori tribe Ngāi Tahu had various uses for bull kelp: the narrow stalk, connecting the holdfast to the blade, was fashioned into a flute; the blade was roasted and chewed; the holdfast stems or stipes were used to make bouncing balls; and wide blades were used as bags for preserving food. Māori made bags called pōhā by splitting open the blades and inflating them.

They produced the bags in large quantities during summer in preparation for the muttonbirding [titi] season. Inflated blades were hung up to dry for several days, then deflated and rolled up. These bags would then be used to store the mutton birds (titi) in their own fat. The outer skin of the blades is airtight, and traps air in the honeycomb-like structure inside each blade. Food preserved inside a pōhā can be kept safely for up to two to three years.

So, to create our massive, floating island style raft, we could either craft it by weaving the strands of kelp together- the simplest method, and the easiest to maintain and repair. Or (/and) we could employ the same method which the Maori used to produce their pōhā to split open and inflate every blade of bull kelp used in the raft's base, greatly increasing its buoyancy and making it a massive inflatable boat. Or, if you want something a bit more advanced and cutting edge, you could simply harvest all of that bull kelp (since it's a brown alginate), industrially process it to extract sodium alginate from it, and use them to mould the floating island vessel's hull out of alginate plastic instead.

Studies have already been conducted into using alginate polymers as building materials, and they've been shown to confer significant flame-, fire- and heat-resistance, along with imperviousness to water. Other substances can be added to the materials to improve cross-linking and/or to produce resistance to fungal degradation. The building materials which can be produced, processed, or treated using alginate polymers include, but are not limited to, wood products; masonry products; wall, roofing, flooring and siding products; and paint products. Furthermore, sodium alginate in the form of a gel may be used as a firebreak to effectively stop the advance of any fires which might endanger the island.

The agar and carrageenan which can be extracted from it are also very useful. As are the hydrocolloids extracted from this seaweed, which have molecules that give viscosity to creams and lotions. In fact, almost all cosmetics include among their ingredients “seaweed extract,” that’s why it is so common that they are incorporated in this class of products. Studies now show that this crop can also be readily used for the relatively cheap, simple and easy production of biofuels such as ethanol, which would doubtless be critical for establishing an industrial base, and sustaining the floating island-raft's propulsion. And to help on the sustainability front too, Cochayuyo is also fully edible; a much loved delicacy in Chilean cuisine.

Cochayuyo for sale in a Chilean food market

Today it is appreciated for its remarkable medicinal properties. It is recommended for weight control, to lower cholesterol, to prevent constipation, goiter, hypothyroidism, heartburn, plus it is an energizer thanks to its vitamins and minerals. It can be eaten in multiple ways: as jam, “empanadas” , “ceviche”, grilled or mashed, croquettes, and even seaweed risottos. But its most common use is in stews like “charquicán”, a typical Chilean dish as well as in salads.

It's commonly preserved by being sun-dried, and harvests can be kept as stock for years- to prepare it in a dish, it simply needs to be softened up by being soaked in water. It also has some of the highest potential yields of any crop, thanks to its extremely fast growth rate, high calorie count, high protein content (enough to replace meat altogether in some native Chilean diets) and an extremely high mineral content (enough to also serve as an extremely useful green fertilizer- albeit also with a very high salt content- meaning that other salt-resistant crops could be grown as well on the island, even without soil).

So, just how nutritious is it? Here's a study of the Southern Bull Kelp's nutritional properties and maximum yields, taken in the waters of Kerguelen Islands:

The approximate composition (% dry weight) of Durvillaea antartica (Cham.) is similar for the frond and stipe, being 28% ash, 3% protein, ca 1% lipid, less than 10% acid-soluble carbohydrate, and 60% acid-insoluble carbohydrate. The holdfast differed by being 22% ash and 66% acid-insoluble carbohydrate. The energetic level of all 3 components was ca 13 kJ (/g dry wt). and ca 17 kJ (/g ash-free dry wt). The relative proportion of the 3 plant components varied little for plants having stipe diameters ranging from 10 to 40 mm, being ca 8, 3, and 89 % wet wt, and ca 10, 4, and 86 % kJ, for the holdfast, stipe, and frond, respectively. Maximal density found in summer was 471 individual plants/sq m, 226 kg wet wt /sq m, and 457 000 kJ/sq m.

To put this in context- most modern countries have cereal yields equivalent to 5 tonnes per hectare. The world record yield for a rice harvest is 22.4 tonnes of rice per hectare; and the present day global production of raw plant calories from staple cereal crops is adequate to feed roughly 10 people/ha. In contrast, even when growing in the wild, largely uncultivated, Durvillaea antarctica in the waters of the Kerguelen Plateau delivers a maximum yield equivalent to 2,260 tonnes per ha, or 350 tonnes per ha in dry weight. In seaweed farms, yields can easily be twice that. And going by the nutritional values of the Durvillaea antarctica samples from the Kerguelen Plateau, that'd equate to a crop yield of roughly 2 million kJ per wet ton. The average adult individual's RDA is just under 1 million kJ per annum; thus, farming Cochayuyo for food, as well as for industrial purposes and for repairing and expanding the vessel, would allow them to feed one person for every single sq.m of cultivation area; 10000 people/ha.

So, just how large do you want your floating island to be, and what sort of population would you ideally want it to be capable of supporting? Durvillea antarctica grows at a depth of up to 15m below the surface; so, let's add sloped skirts around the 'coastline' of our island, going down to this depth, to provide an anchoring area for their holdfasts and serve as our cultivation area; roughly 30sqm for every meter of 'shoreline'.

If you do, an island the size of a large cruise ship would also produce a Cochayuyo harvest large enough to support a population of more than 20,000 people; an island the size of the Project Habbakuk Aircraft Carrier would be capable of growing at least enough Cochayuyo to feed roughly 84,000 people indefinitely. And the cultivation area could easily be increased five-fold or more by changing the shape of the island's 'coast', giving it a leaved or petaled shape, thereby increasing your maximum sustainable population until it's larger than Iceland's population.

Or, to present a less dystopian best-case scenario, to support a population of around 25,000 people in relative comfort, even with only 6% of the annual cochayuyo harvest used for food, and the vast majority of the harvest utilized for other purposes such as the creation of biofuels, biopolymers and bioplastics, skin products, cosmetics, and fertilisers to grow other crops in greenhouses on-board. How does that sound?


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