Scenario + question: So for the purposes of this question lets so that there is a canyon much like the grand canyon but about 50 miles deeper, so the bottom is always in shadow and it has no observable bottom (but there is one). What kind of gas would it need to be filled with for floating islands to be possible?

Requirement: This is in a world that has natural magic (as in occurs naturally and by itself), however I would like to make it at least kind of scientific. The gas might be so dense that it appears on on the bottom of the canyon in a liquid state, which is OK. The rock may also have another gas trapped inside (e.g. helium or oxygen), however it needs to be solid enough for a building to exist on the top (more like huts because of the time period).

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    $\begingroup$ An idea for your side question, think airboats in swamps. They don't use rudders and drag, they just change the direction of thrust to turn. $\endgroup$
    – Virusbomb
    Sep 29, 2017 at 15:03
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    $\begingroup$ If you have magic, don't try to make it scientific. By it's very nature magic is unscientific. If you want floating rocks all you need to do is say "There are floating rocks. It's probably magic but we don't know for sure." $\endgroup$
    – sphennings
    Sep 29, 2017 at 15:09
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    $\begingroup$ Related: Naturally-ocurring Airborne Floating Islands - is it possible? $\endgroup$
    – Mołot
    Sep 29, 2017 at 15:14
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    $\begingroup$ It's convention to wait at least 24 hours before accepting an answer so people from every time zone have a chance to weigh in. $\endgroup$
    – sphennings
    Sep 29, 2017 at 16:51
  • $\begingroup$ I confess, my first thought was to host a banquet featuring "Mexican onion and bean soup", then wait 12 hours. You might have problems with the rocks dissolving though. $\endgroup$
    – pojo-guy
    Oct 1, 2017 at 2:19

4 Answers 4


Scientifically, this can't be done. "Floating" is nothing more than density of object A being less than the density of object B. Hot air balloons float because the density of air inside the balloon is so much lower than the air outside the balloon it can lift its own weight and a bit more.

Therefore, your gas would need to be more dense than the rocks. At that density, whether a gas or not, you could walk on it (and, to answer your side question, boats would work just fine... assuming they could move through something more dense than rocks....).

It's possible to address the question as rocks filled with an offsetting gas (e.g., helium), but it would have to be very little rock to a lot of helium due to that "their own weight and a little more" statement I made about hot air balloons. Look at how much hot air is required to lift a nylon bag. Now pretend that bag was as dense as rock.

Therefore, let's assume your planet has a type of rock that forms thin, difficult-to-break shells as they rise through magma, trapping gas inside. In real life, that gas is never helium, but let's stretch reality for a moment and say that it is. Your floating islands would be fairly large, paper thin (though difficult to break) and almost entirely made of this material. Let's say it shimmers like glass. It would look cool. But this is as close to science as you can get (and there's not much science here).

  • $\begingroup$ Take a look at pumice. $\endgroup$ Sep 29, 2017 at 23:46
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    $\begingroup$ @WhatRoughBeast, Pumice isn't a bad idea, but it floats in water, not a gas. Even it hasn't a low enough density. It's a good idea for the concept of a shell, other than it's quite brittle. Good catch, though. $\endgroup$
    – JBH
    Sep 30, 2017 at 2:40
  • $\begingroup$ In Weiss and Hickman's "deaths gate" books, there is a type of coal that builds tough but light weight pumice like shells with extensive hydrogen bubbles. $\endgroup$
    – pojo-guy
    Oct 1, 2017 at 2:21
  • $\begingroup$ The other problem with porous rocks like pumice is that they’ll slowly fill with the heavier gas and sink anyway. Might be a cool plot device, but hard to make happen with science alone $\endgroup$
    – Dubukay
    Aug 18, 2019 at 19:15

In order to have floatation, you have to apply buoyancy law, a.k.a. Archimede's principle: the density of the fluid has to be bigger than the density of the body.

Gases are tipically 3 order of magnitude less dens than solids, so there is no way you can have a massive rock float in a gas. You can still have a hollow shaped rock float, as long as the average density is lower than the gas. Basically a rocky ballon.

Sulfur Hexafluoride is a pretty dense gas, so you can go for it.


Edit I have thought of a better way, but will keep my other suggestion below for amusement.

If we abandon equilibrium conditions it makes things hugely easier, if the system is not at equilibrium there would be far more scope. So if your canyon has a high continuous and steady gas flow flowing out of it upwards and it is slightly "v" shaped in cross section then rocks might float in the same way that humans can float in an air stream like this:

enter image description here

Original post It depends on how bizarre you are prepared to let your environment be. If you were hoping for it to be a human habitable world even approximately, then I would say not a chance. It’s a tough call in any environment, even excluding human habitability. But let’s see what’s possible. My answer to this recent question might be of interest and I guess if you’re using magic then you won’t mind playing a bit fast and loose with the physical parameters of the world?

At around 25 degrees C Xenon gas has a density of around 3g/cc at 203 MPa (around 2000 atmospheres pressure) seep84. Although this must be close to its condensation point at this temperature and pressure as the liquid density is only 3.1c/cc. That should be enough for your floating rocks as a solid rock is not that compressible and you could pick some nice light rocks from this list.

But this world would be beyond weird. Although technically a gas it will be so dense it that walking though it would probably feel more like swimming in treacle or mercury than walking.

A phase diagram for xenon would have been useful, but I couldn't find one.

See also https://en.wikipedia.org/wiki/Xenon

Perhaps an easier answer would be to suppose some form of magnetic levitation along the lines used in the film Avatar...

  • $\begingroup$ Cody from Cody's Lab was able to float a light weight metal on high pressure Xenon gas, it's not much of a stretch to try rocks which are more like 2g/cc $\endgroup$
    – Samwise
    Sep 29, 2017 at 21:41
  • $\begingroup$ @Samwise Interesting. If you forget rocks and go for the lightest metal Lithium density ~0.5g/cc and then look up the values in the tables in my link above (page 81) this should work for Xeon at room temperature at a “mere” 6 MPa or 60 atmospheres. A high pressure, but it should be obtainable in a lab with the right kit. Of course the experiment would have needed a lot of very expensive Xeon and some fancy pressure vessels. Me thinks it woud have been easy to fake it as well… Do you have any links for this? I think the difference between 0.5 and 2 g/cc in huge as we're taking about gases here $\endgroup$
    – Slarty
    Sep 29, 2017 at 22:56

Leaving out a few technical impossibilities, such as gasses not separating, and that a canyon (or any hole, like a crater) much deeper than 10 km/6 miles is unstable for the same reason that mountains higher than that are unstable, consider aerogels. Graphene aerogel is reportedly lighter than air: https://www.extremetech.com/extreme/153063-graphene-aerogel-is-seven-times-lighter-than-air-can-balance-on-a-blade-of-grass

The problem is that (at least as I understand it), the graphene aerogel is porous, so it's permeated with air and so doesn't float. But with some fancy bioengineering, say, you could manage to grow closed-cell aerogels containing hydrogen, perhaps even with lower than ambient pressure. The aerogel "tree" would even have a "bark" surface, so you could walk around on it.


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