So, on a previous question of mine, someone answered that it would be feasible to build a city on a waterborne floating island using a pumice layer below the ground.

That got me thinking about the possibility of having a city built on a floating island, but airborne.

Now, I know that there have been questions about floating islands already... but those implicated major geological catastrophes projecting rocks into the air, technological or magical human intervention and atmospheric gases different than our own.

But my question is different: Is it possible to conceive of an imaginary rock that would float on the air of an Earth-like atmosphere, some distance above the ground? I know this seems impossible, but having a rock that floats on water also seems impossible and we have pumice.

I don't want real world examples (they don't exist), only to know if such a rock could be theoretically possible. A geological material that could form floating islands in the sky where the building of cities would be possible.

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    $\begingroup$ Pumice floats on water because it contains pockets of air. To get a rock to float on air, it would need to contain pockets of something much less dense than air. Maybe that's a starting point? $\endgroup$ Jan 22, 2017 at 21:48
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    $\begingroup$ If your island had a big block of superconducting material inside (and more in the ground) it's entirely possible that it would float in the air, although I can't say how high you'd be able to get before the effects diminished (magnetic fields and the inverse square law and all that) $\endgroup$
    – Samwise
    Jan 22, 2017 at 22:30
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    $\begingroup$ Superconductors have an innate ability to strongly repel any and all magnetic fields from their interior, this means that when a mass of super conductor moves over a magnetic field, not only will it be lifted into the air, it can even be locked in place as if by some invisible force field (it's called "flux pinning") here's a good video demonstrating the effect $\endgroup$
    – Samwise
    Jan 22, 2017 at 22:55
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    $\begingroup$ @MarkGardner that's not how it works. Atmosphere gets thinner the higher you go, and at some point you reach point where your buoyancy is 0. $\endgroup$
    – Mołot
    Jan 23, 2017 at 14:33
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    $\begingroup$ @Mołot not if your weight is negative. Alternatively, they might have hung around, and been smashed by meteors $\endgroup$ Jan 23, 2017 at 14:49

10 Answers 10


Consider a biological 'assist'

You have your chunk of pumice-like rock, with plenty of pores, holes, and tunnels. Within these is a thriving bacteria. The bacteria is particularly attractive to a fungus that can grow without direct sunlight.

The fungus' output is a gas that fills a bladder or bubble within it. This gas is lighter than air and as the fungi fill the bladder, it begins to lift the soft-stone structure slowly until it reaches a certain atmospheric pressure. At the equivalent of 5,000m, the gas is equal with the atmosphere and the island stabilizes with respect to altitude. This does not mean it will stay still.


You will still have a lot of wind. Do you want this? You can have a community develop on the 'island' whose weight presses it down, and the response of the gas-bladders lift it back to that level. However, winds will push it every which way at the 5,000m altitude.

You can run into things. The choice of 5,000m is somewhat suitable for humans. However, this is roughly the altitude of base camp for Mt Everest, so there may be mountains struck. Your next question could focus on how to stabilize your now-floating mountain. I look forward to that.

EDIT with ADDITIONAL INFORMATION: I am not sure your intended use of the island or the type of city you want, but a city might weigh 10m tons. I am going to go with a compact, radial city, in order to address your question although there is a LOT of speculation and variables.

enter image description here

Warm gas like methane filling sacs within your fungi will lift the city until it reaches equilibrium with the atmosphere at around 500m height. This means that even if the fungi continue to produce more gas, your city will max out at around 500m. The weight of your city presses down, but your CH4 -filled fungi press up. Atmospheric conditions are not stable, so there will be a lot of ups and downs - make sure your denizens and engineers are comfortable with that.

Note: I am not an artist / Note2: There are too many unknowns for precision math, but this could work for your story at a minimum.

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    $\begingroup$ Great answer, and very imaginative! Thank you! However, do you have anything that would address Molot's objection, that using floating gas would mandate very thin walls on the rock, that would burst out with high altitude pressure? $\endgroup$ Jan 24, 2017 at 13:45
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    $\begingroup$ @PedroGabriel - I think the bladders on the fungi can be extraneous to the pumice rock; that is to say, they are outside of your soft rock, so they can be as varied and populated as much as you like on the outside of the rock. The gas should meet equilibrium of the atmosphere at the elevation you prefer. So even if you have a million extra fungi sacs, if they're equal at the height you prefer, it is okay: just more stabilization. $\endgroup$
    – Mikey
    Jan 25, 2017 at 3:29
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    $\begingroup$ Won't the rock fracture before it can lift.. it'll be too weak to contain the high pressure gas which could lift it $\endgroup$
    – Innovine
    Jan 28, 2017 at 14:05
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    $\begingroup$ You guys are WAY overestimating the lifting power of any possible lighter than air gas; as one of the answers below point out, you get about a kilogram per cubic meter of gas. So imagine a 1km balloon for every ton of city. (not including the island) $\endgroup$
    – Seeds
    Feb 6, 2017 at 22:25
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    $\begingroup$ While I did notice the lack of a hard science tag, the question ALSO asked if it were theoretically possible, to which the answer is NO. The only way to make it work is with anti-gravity rock. Air doesn't weigh enough. $\endgroup$
    – Seeds
    Feb 7, 2017 at 15:14

Not really. You would need a porous rock filled with hydrogen or helium. Hydrogen can lift bout 1.1 kg per cubic meter of its volume. This means you would have to have about 1kg of rock holding 1 m³ of gas. This means bubbles would need to have really, really thin walls.

Now, thin walls are not out of the question. No problems when pressures are equal. But then your rock would rise from were it formed. Up there, pressure is lower. Thin walls and pressure difference would make bubbles burst, release hydrogen. And all you get is shattered pumice.

For magnetic lift, there is another problem. Permanent magnets don't hold its magnetism for too long, and no known substance could keep superconductivity at outdoors temperatures — as far as we know now. Our knowledge of superconductivity was shattered few times and we have superconductors so hot that 20 years ago we knew it's impossible, but then, now we can only speak from the point of view of the knowledge we have now — and superconductive pinning without power supply doesn't seem possible nowadays.

  • $\begingroup$ A room-temperature super conductor and an abnormally strong magnetic field would do the trick. Just because we haven't seen one doesn't mean they're not possible. $\endgroup$
    – jorfus
    Jan 27, 2017 at 22:59
  • $\begingroup$ The big problem with magnetic lift is that you can't have lift with permanent (passive) magnets alone. It's a consequence of Earnshaw's Theorem, and the usual loopholes (using also diamagnetic materials, or active controls) aren't applicable here. $\endgroup$
    – LSerni
    Jan 27, 2017 at 23:25
  • $\begingroup$ LSerni, could you please explain why diamagnetic (or superconductive) levitation is not applicable here? $\endgroup$
    – Alexander
    Jan 27, 2017 at 23:39
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    $\begingroup$ @Alexander, sorry hadn't seen the comment - that's because we want passive operation, and passive magnetic levitation is simply not possible. However cleverly you place your magnets, the configuration will always be unstable. You need some stabilizing effect apart from simple gravity, and that must be active - be it a gyroscope, a PLL, ultrasonic feedback. The only "passive" means would be chains fettering the island to the ground. Finally, diamagnetic stabilization would require a magnetic source and a thick shield of e.g. bismuth above the island. $\endgroup$
    – LSerni
    Jan 24, 2018 at 13:49

Imagine an extremely fibrous vine. Now imagine that this vine likes to grow on pumice, which it roots deeply in. Finally, imagine that this plant's metabolism, somehow, produces an excess of hydrogen, which it stores in hollow leaves. Since the vine gets no nutrients from the roots anyways, it would not be limited in length. Its strength could be due to structures similar to carbon nanotubes.

Not exactly a floating rock, but scientifically plausible. And, by putting the buoyant gas-bag above the low-density rock, you avoid the problem of building on top of the rock causing it to flip over.


Option 1: The solar system design

Why can't you have a small scale solar system as a planet?
If you thought of our solar system as one planet, it would be very similar to what you're requesting.
Instead of having a crust, I don't see why a planet couldn't be straight core... with no inhabitable land mass attached to the actual planet. From here, all the "islands" are just land masses that are in a low orbit around the core.
As for an atmosphere, the ISS is technically in the earth's atmosphere, so there are basically 3 options.
1. Having your inhabitants capable of surviving in an EXTREMELY low atmosphere environment, meaning that they don't have the same gas requirements as anything remotely human.
2. Designing the base planet to have an incredibly strong atmosphere, that would be too dense for long-term living at the surface, but be perfect for living on the fringe, where the orbiting islands are.
3. Give each orbiting island it's own atmosphere, limiting inter-island travel, but being easy to communicate to readers.

Option 2: The helium generator

This one's a bit of a stretch, but if you have a very small nuclear fusion reactor (read star) at the middle of each island, which had branches/corridors running through the island, you would have an island lighter than air, but you would need an incredibly air-tight center, and the island would have to be long and flat.

Option 3: The dense atmosphere

We all know that tungsten hexafluoride is the densest gas, at about 13 grams / liter, and we should all be familiar with aerogel, the solid made out of silicon dioxide and air. In theory, a large enough island, composed primarily of aerogel, or something similar, could work when sitting on top of an "ocean" of tungsten hexafluoride. This would look like boats of light solids on top of heavy gasses, which could work.


As far as I know, all of these are plausible, if not extremely rare. They are also all longterm, meaning you don't have to be worried of magnetism fading, etc. I like option 1 the best, but in a properly engineered system, all of them would work. From a purely dynamic standpoint, option 3 would be the closest analog to a boat (read floating island) as it would be an actual platform resting on what it perceives as a solid base, as opposed to counting on some other property (orbit/lift) to maintain the floating aspect of the island.

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    $\begingroup$ Welcome to the site. Note that the first option couldn't work: atmospheric drag would slow and then stop the islands unless they had ways to propel themselves - the main physical different between islands and space stations. While I love your third option, the OP also said they weren't interested in atmospheric gases different than our own. Still, creative, and welcome! $\endgroup$
    – Zxyrra
    Feb 3, 2017 at 0:30
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    $\begingroup$ being an atmosphere half full guy, I don't see why the atmosphere couldn't be half tungsten hexafluoride, aka what the island was sitting on, and the top half be a mixture similar to ours... as to the first option, subsection 3 might be the best variant on option 1... but I feel it could work if the inhabitants could live on extremely low atmospheric pressures... as in like... ISS atmospheric levels. But thanks for the welcome! I like it here, the community is very nice, and the questions are incredibly thought provoking $\endgroup$
    – Sharkn8do
    Feb 3, 2017 at 4:33

Not naturally, but yes. And for gengineered values of "natural"...

You can have substances quite resistant and lighter than air, but not by very much. Evacuated aerographene has about 16% air density, and can withstand considerable pressure. Every cubic meter of aerographene can lift about 800 grams in air. You can cover it with air-tight silicon or graphene foil, so that it stays evacuated, reducing its buoyancy to about half a kilogram. Non-evacuated aerographene is heavier than air and would sink.

The reason why we fill balloons with hydrogen instead of vacuum is that we need something inside to counteract the outer air pressure and prevent the ballon from collapsing with a thunderclap. But if the "balloon" is filled with aerographene, that is enough. Actually, an even lower "density" can be achieved with a hollow evacuated aerographene ball (spherical to better withstand the pressure) with an airtight outer shell. But there would be air between the balls, so every cubic meter would be about 25% air if we use equally-sized balls, 75% hollowed aerographene and some shell material. On the other hand, the material resistance is high enough that we might be able to use hollowed-out cubes instead of spheres.

But we're pushing against an absolute lift limit of one kilogram per cubic meter (the lifting power of vacuum), so we'll hit the point of diminishing returns pretty fast.

Small spheres would have the advantage of being manufacturable by biological constructs (you'd need impressive genetic engineering skills). You could cultivate (or mine) the spheres of levity and believe they're natural.

To be able to lift one ton per square meter, the "island" would need to be at least two kilometer deep. Actually more than double than that, because you want to have more mass as ballast below the island than above, or the island will tend to capsize. Also, "upper" air will be less dense and provide less buoyancy.

If you're willing to have a thinner upper crust and employ a lot of aerogel for the upper buildings, and use flimsier sealants in the island's core, you can have a reasonably thin floating island. Which brings us to the problem of steering it and surviving storms and the like.

  • $\begingroup$ I really like the idea of mining "spheres of levity". That's an amazing pun combining "levitation" and "humor or frivolity, especially the treatment of a serious matter with humor or in a manner lacking due respect." $\endgroup$
    – Rob Kinyon
    Jan 24, 2018 at 13:38

For a naturally ocurring floating island, we need a naturally ocurring:

  • A lighter than air gas (methane, near vacuum ambient air)
  • A membrane impermeable to gases ( guanine like in swim bladders, fullerene, ...)
  • A material strong enough to resist the pressure differences (to avoid implosion or explosion) like diamond, fullerenes, ...

What could have happened is that similar to the diamond formation process, a rock made of carbon nanotubes imprisons bubbles of very hot air under very high pressures deep down in the mantle of the planet (bubble diameter varying from mm to m of diameter).

As these rocks move up to the crust, they cool down and the carbon based material solidifies and imprisons the gases. As the gases cool down, it turns the inside of the bubble into an effective void.

In the process, they would collect ammonia rich materials (more on this later).

Billions of years later those rocks reach the surface and the local sapient specie(s) notice that a certain rock, if let free from the other rocks, will float. If caught in nets, they make it very easy to carry heavy loads over long distances.

As extraction progress, dear devils from the younger generations start experimenting climbing on the bigger rocks and playing with floating and how to direct the rocks with artifact. At some point they will realise that what used to be a problem for their parents (breaking the rocks makes them fall) can be used to avoid going too high, they might even master how to do it to control rising / falling.

Then someone will notice that a rock which was burst with a small crack can be made to float again if filled with ammonia gases.

Because of the huge amount of naturally occurring ammonia, they would have developed a rudimentary absorption fridge wikipedia, then a local equivalent of Da Vinci will design a way to control the amount of ammonia which goes into those huge balloons made with this very light material (basically a huge bubble burst but the rest of the material still contains a lot of small bubbles so it is still very light) and thus adjust the floatability of the system. Attaching those ammonia filled balloons to the 'almost' floating island will allow them to float and control altitude.


Yes it might be possible.It can be achieved in two ways.

  1. Uing quantum levitation

    If the island is somehow locked into its position by quantum levitation effect against the earths magenitic field it can stay in the air.But considering the weak force of the earths magnetic field when compared to the earths gravity this might be hard to achieve such effect.

If you had a really really big magnet, whose field extended over such a large region that the Earth’s field changes noticeably over that region (you might need another Earth-sized bar magnet), then yes, a noticeable force can be produced.

this link explores that option indepth

  1. Antigravity from Supergravity

    Antigravity like effect could be achieved can be achieved from supergravity called as unified supersymmetric point-particle quantum field theory.

Some particular versions of this theory not only contain the "usual" atractive graviton, a spin-2 particle, but in addition a so-called graviphoton is predicted (1). This graviphoton is a spin-1 vector field, interacts with mattar at the normal gravitational strength, and behaves generally like a massive photon. The fun thing about it is that it can give rise to attractive and repulsive forces. The repulsive forces feature can in principle give rise to anti-gravity effects.

these links explore these option indepth

https://physics.stackexchange.com/a/60349/40095 http://www.sciencedirect.com/science/article/pii/0370269379904635

If anyone of the above effects occur in nature then there possibility of natural floating island.



Islands floating on liquid

On geological timescales, things crumble, especially large things. This is why you don't actually see islands of pumice, and there's no large patches of pumice floating about. In the end, the pumice crumbles: the gas escapes, the rock sinks.

Now, given the right circumstances, some freak vulcanic eruptions or something, a large pumice island may form, but given some large waves or strong currents, it will break apart, and eventually it will crumble.

In a more general 'any solid floating on a liquid' sense, over time, and on a large scale, substances arrange themselves on density, the least dense material on top. So, given enough of any solid that's less dense than a liquid, it will generally form a thin layer above that liquid (a bit like the Earth's crust).

However, there is one possibility for solids floating on liquids permanently:
The sweet spot where there's not enough of the solid present on the planet to cover the entire planet in a layer, you'll end up with patches of this solid floating over a liquid planet-sized ocean. Example: islands or rock/sand floating on an ocean of mercury.

Islands floating on gas

The problem with floating on gas is that there aren't any solids light enough to float on normal air without pockets of gas. But that's where the crumbling comes in again.

There are two ways to fix this

Denser air. There are some gasses out that that are pretty dense. And you can ramp up the density even further by increasing the pressure at which point light solids might be able to float on (or rather somewhere in, at a certain pressure) the gas. (This actually is a thing).
Again, you'll have to be in the sweet spot where your solid doesn't just cover the entire planet but forms patches.
But I don't see a planet like this supporting intelligent life.

Counteract crumbling. The only way I can think of is some kind of organism which heals any damage and inflates bladders, like in Mikey's answer.
This actually seems like a decent survival strategy for some micro-organism, to build their own sort of zeppelin and (like pollen) drift in the wind.

These organisms could produce methane (about half as dense as air), or even better: hydrogen (~1% the density of air). All these organisms need are CO2, water, preferrably calcium (from sea-spray?) and some energy source (photosynthesis), they could form some coral-like (calcium carbonate) structure filled with hydrogen bubbles (this would require large bubbles though).

Also, it would help if the atmosphere were a bit denser than here on earth, if the air pressure were 2.5 times the current air pressure, calcium carbonate would almost float, so the bubbles can be a lot smaller. A higher air pressure probably also has lots of other implications.

These islands would need to be flat (have a large area for photosynthesis) and probably contain some large cavernous hydrogen pockets somewhere on the inside (potentially formed by decay of calcium carbonate walls on the inside).

Having floating islands of hydrogen filled with hydrogen also opens up the possibility of spectacular explosions and crashes.

Living on an island like this requires very light construction (and/or compensating with hydrogen/helium balloons); not covering too much of the island, as without photosynthesis it would die; only very controlled fires; no drilling; and a good balance to prevent tipping.


Okay; first things first, I'm no geologist, but I think it's not impossible. First of all, you would want an extremely strong and extremely light pumice-like material (some kind of aluminium alloy, maybe?) to build the actual island itself out of, and you would also probably want a fictional gas significantly lighter than hydrogen to fill the internal bubble honeycomb structure.

  • $\begingroup$ Even vacuum isn't significantly better than Hydrogen. $\endgroup$
    – Seeds
    Feb 6, 2017 at 22:19

Ok, there are many answers how could it be possible, but the main question - will it be stable enough to build structures on it? We can imagine some stable point for a floating island, where it will neither rise, nor fall down, at this point it's pretty stable and will not turn other. But each building will add weight, and island will loose it's attitude to the point it won't be that stable, and could turnover.
Also, we DO have a floating rock in our sky. The Moon. The thing is - it's always falling, but constantly misses the Earth.


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