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[Here's some writing. It will explain some aspects of the abyss in a way I can't otherwise.]

"It's beautiful", uttered Ivan, in a rare moment of appreciation. "Rocks and platforms and chunks of land all floating in an endless abyss of the blackest black. This one's different from the others. If we jumped off this ledge, we wouldn't fall too far."

The lithomancer, Tumor, glances up.

"The air here is breathable, but so thick you can almost swim in it. Seeing as how you're a pyromancer, Ivan, you could probably propel yourself regardless; however, Rhys and I will need to take advantage of its odd thickness."

Rhys jumps into the abyss. She travels a very short distance before halting in the air. She makes a breaststroke motion and moves forward a bit.

"It's amazing, I'm completely weightless. Come on in, the chasm's fine."

The other two jump into the rift and begin trekking across.

The abyss is deep underground but not deep enough to reach the planet's (presumably hollow were this the case) core. A waterfall on a distant floating mountain evaporates into the abyss, filling it with clouds. Occasionally, movement is seen. Never is wind felt, however. Perhaps they're not alone?


So, how can I make this theoretical abyss possible?

Some details and elaborations about the chasm from the story:

  • It's not a little chunk of space, as air is not only breathable, but thick enough for reasonable propulsion.
  • Nor is it a hollowed core at the center of the planet.
  • It's big enough for weather and huge chunks of land resembling small mountains to be dwarfed by its vastness.
  • As it's an underground chasm, there's no wind - the air is still.
  • No, by "the air is thick" I do not mean the cavern is flooded with water. Things like clouds and pyromancy would be rather difficult in that situation, not to mention any buoyant force on the landmasses and such. It means that the air resistance is high enough to give a bow an effective range of no more than about 40ft and allow for something similar to swimming to propulse oneself.
  • Bioluminescent molds and lichens provide enough light to make it appear as if it were daytime if not for the infinite nothingness beyond the clouds. This is only really notable in pointing out the abyss can support life.
  • And, of course, gravity just sort of vanishes past the ledge. It's a slow transition, but only slow enough to get a bit of an arc when throwing something or jumping off the ledge.

There are two major questions:

  • What could be the reason for a chasm so large (potentially miles/kilometres across in all directions but up) existing underground?
    • Furthermore, chunks of land ranging from tiny flecks of stone to small mountains were left despite this.
  • And more importantly, why would it have some of its properties, such as super-viscous air and near absence of gravity?

Answers are preferred to have a scientific basis, but given the high fantasy setting and frank implausibility, magic is also acceptable - especially if it has a good excuse (err, "lore reason") to be so.

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  • $\begingroup$ See the recent question "How would a floating island work" and consider expanding upon those ideas. $\endgroup$ – Zxyrra Dec 4 '16 at 4:04
  • $\begingroup$ @Zxyrra Ooh, nice. Still, it's less "how get rocks to float" and more "how get EVERYTHING to float". $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:06
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    $\begingroup$ Oh, and, as a general rule, sorry for the seemingly contradictory [science-based] and [magic] tags both being here. Not sure how else to point out that "Science is preferred but magic is also okay especially with a good narrative". $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:07
  • $\begingroup$ @kingledion "Effective" range. Granted, in real life, a bow's effective range is less due to air resistance and more due to the arrow going THK into the ground, which isn't an issue in a gravity-less abyss, but still, I don't think you can shoot a bow too far and have it do any real damage under normal conditions (negating speed added due to gravitational acceleration). $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:10
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    $\begingroup$ @Zxyrra After a quick refresher on relativity theory, I must say this is an excellent idea. (For those less physics-inclined: My reasoning being that objects in a gravity-less container accelerating at 9.8 m/s^2 would behave the same way as objects would under our 9.8 m/s^2 earthly constant gravitational acceleration.) While this isn't perfect, it just barely sounds plausible if you look at it from a certain angle and draw connections and make leaps of logic that may or may not make sense and build from there. Maybe this particular chunk should be on Physics.SE. $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:51
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There are lots of reasons for giant caverns

Popular ones include:

  • Karst topology. If the area is made out of limestone, then water will slowly dissolve the limestone over time. Enormous caverns exist all over the world in limstone landscapes.
  • Salt Domes. Giant evaporated salt deposits, such as the one left by teh Aral Sea, or in the Bodele depression, will over geological time become buried. Then, the salt in them could be partially dissolved by water, or they could trap hydrocarbons that are then drained off, leaving caverns.
  • Empty Magma Chamber. Around volcanic hot spots or fault lines, magma chambers can form. If the lava then later drains out of them, caves can form. These caves can be very large and VERY deep underground, and might be the best fit for your story.

If you want lots of bits of land mass deposited in this cave, the reason could be as simple as a partial cave-in depositing thousands (millions?) of tons of rock into the cavern.

Super dense air is not possible

If the air is thick enough to prevent a bow from shooting 40 ft, then the air is probably deadly. Gases have a maximum density. To get them more dense than that, they would have to be a liquid. I don't know of any gas that is dense enough to stop an arrow like you said; and in any case, any gas significantly denser than air would displace oxygen and suffocate people.

Super dense air is only possible by magic.

Gravity is a property of matter and cannot be turned off

Gravity can be viewed as an impression in a mattress caused by a large object, like a planet. Anything on the mattress that is close enough will 'fall' into the depression caused by the the heavy object. The effects of gravity propagate equally in all three spatial dimensions. On the surface of a planet (that is a sphere) there is no way to have gravity in one place but less gravity in another.

A gravity-less zone on the surface of a spherical planet is only possible by magic.

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  • $\begingroup$ So, basically, "Here's how the geology would work. However, I'm just going to say the other parts are physically impossible and not even going to try to discuss possible magic solutions aside from suggesting they're likely the only feasible method." I do like the geological suggestions, so I'm not downvoting this, but basically brushing off the most important part also demands I don't upvote this. $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:26
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    $\begingroup$ Oh, and as an aside, you TOTALLY can have different levels of gravity on a planet! Relevant xkcd. $\endgroup$ – Papayaman1000 Dec 4 '16 at 4:27
  • $\begingroup$ @Papayaman1000 Its hard for me to give a lore based reason without knowing the lore of your world. Asking people to make up lore for your world is off-topic at Worldbuilding. In any case, you asked for scientific assessment and I gave it. I think you should make up a magical reason to explain the last two, and that it shouldn't involve the density of air or gravity. Maybe magic removes inertia from all arrows, or accelerates all objects up at 9.8 m/s^2. $\endgroup$ – kingledion Dec 4 '16 at 5:09
  • $\begingroup$ Very well, I suppose you have a point. The "lore reason" part is less "make up why magic works for me" and more "please explain just what effects this magic would have should you take this route". Oh, and, the whole "effective range of a bow" thing was less a desired effect and more a frame of reference for how objects in this abyss would behave in general (objects would slow down and stop rather quickly, as if facing high air resistance, rather than drifting infinitely without decelerating, like space). $\endgroup$ – Papayaman1000 Dec 4 '16 at 5:24
  • $\begingroup$ A note about super dense air - the pressure of a gas is proportional to its density, so super dense means a lot of pressure. Something has to be containing it, so unless the characters went through an airlock to get there, you need magic to keep the air in its super dense state. $\endgroup$ – Rob Watts Dec 7 '16 at 23:08
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I haven’t done the math on this (but (a) you specified and not , and (b) you’re allowing ):  The chasm lies under a deposit of some substance that’s many times denser than rock (neutonium?).  It exerts an upwards gravitational pull on the space immediately below it; with enough mass, it should be able to create a region where the gravitational field is zero (or infinitesimal).  Unfortunately(?), that also produces a higher than normal gravitational field on the surface above the deposit (i.e., above the abyss), but (handwave handwave …).

        cross-sectional view of the planet

It’s hard to image such a structure occurring naturally, or being stable if it does exist.  It would probably collapse under its own weight.  So enter deus ex machina… it’s a constructed artefact (perhaps by a lost civilization, or by aliens).  (The builders might still live a mile lower.)  To support this notion (pun intended), I’ve added a second layer of the substance below the chasm, with pillars holding up the upper layer.

This doesn’t address the viscosity of the air.  But, if you stipulate that the chasm is accessed by tunnels from the ocean floor, you can argue that the atmospheric pressure is much higher than that at the surface, and that’s a start.

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  • $\begingroup$ Would this not also make the gravity on the surface above the deposit/structure/whatever (henceforth a "smeerp") much higher downwards, as in, basically doubling the weight of anything in that region? The smeerp would ideally not do that. Still, a novel idea that I never thought of. $\endgroup$ – Papayaman1000 Dec 4 '16 at 22:55
  • $\begingroup$ Well, I mentioned that (in passing). If the smeerp is deeper underground than my drawing suggests, its effects at the surface might not be quite so severe. And if you make that part of the planet inhospitable (e.g., mountainous, or an ocean), people might not notice. $\endgroup$ – Peregrine Rook Dec 4 '16 at 23:04
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    $\begingroup$ It's a bit too late to change the topology above the cavern - this is an RPG situation, after all. A major goal is to make sure the smeerp's effects are, for the most part, localized (other areas connected to the cavern could have a noticeably different gravity, but not quite the same full-on free-float), and to bury this deep enough to have the same effect throughout a miles-in-all-directions cavern but not the surface would make it so deep that I might as well hollow out the core of the planet for the cavern (which is not ideal). $\endgroup$ – Papayaman1000 Dec 4 '16 at 23:08
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Anti-gravity isn't really what you are looking for. Gravitational nulling is. The precise alignment of a large moon, close enough and in coordination with a small enough sun and the right density planet could create a temporary lagrangian point in the chasm.

If actual anti-gravity is what you want, then it would be as hard to find with magic as it would be with science, I surmise, because if gravity is brokered by quanta with spin-2 bosons, then they are very hard to detect (we still haven't), and certainly very, very hard to manipulate. Interrupting the local brokering of space-time deformation by gravity might produce the effect of floating, but probably would have some pretty unpredictable results. Without gravity, time itself would speed up for the viewer caught in the null gravity field. That is straight up Relativity theory. Without gravity, the concept of NOW becomes even more questionable than it already is. NOW, as a concept, isn't covered in physics like you might think. It has special significance to us, but it doesn't exist in the formulas that physics uses to describe physical nature. The person caught in a zero gravity field would experience the same NOW with others in it, but the NOW of the outside world would be racing past.

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  • $\begingroup$ W... wow. Time is relative. How did I forget? Except wait, that's in relation to speed, not gravity. $\endgroup$ – Papayaman1000 Dec 7 '16 at 21:38
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    $\begingroup$ It is related to gravity as well. Gravity curves space-time. You aren't going anywhere on the surface of a heavy star, but time is warped. Outside the presence of gravity, where space-time has no warping, you get a similar effect. Though I think I got it backwards. Time outside of your anti-gravity zone is frozen. If you spend an hour in there, when you return, it is the same time as when you entered. So you could go in wounded, heal up, and come out and it looks like super healing. $\endgroup$ – Pete Mancini Dec 13 '16 at 18:02
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    $\begingroup$ That would actually be an incredibly cool effect, or at least, a side-effect I don't mind. Plus, as the transition into nongravity would be gradual, albeit still quick, it would circumvent the issue of your entry and exit limbs aging days ahead of everything else that the "time wall" idea has. And yeah, gravitational effects on time makes sense. If someone falling into a black hole would appear frozen and super redshifted... though astronauts don't age super quickly and appear blueshifted when they go to space (though that Lovecraftian idea would be simply paramount)... $\endgroup$ – Papayaman1000 Dec 14 '16 at 3:46
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There had been a suggestion of using an air-xenon gas mixture at high pressure to create an environment where buoyancy was sufficient for humans to "swim" in air. Such an environment would appear to lack gravity in the same way that being underwater seems to be weightless. It's not, but buoyancy gives lift.

The drawback to this air-xenon gas mixture was that it was an anaesthetic, so while humans could float in it they would be unconscious while doing so. While this isn't the recommended way to achieve the effect the OP wants it does provide a possible mechanism -- provided any possible adverse effects from a suitable gas mixture can be overcome.

If the abyss was actually inside a gigantic pressure chamber, then a suitable gas mixture could be contained there sufficient to create the buoyancy for humans to float as if they were weightless. This mechanism would need to be ramped up considerably more so to ensure that rocks could also float in abyssal air.

In a set-up of this sort, perhaps magic can operate in two ways. To overcome the anaesthetic properties of a high-pressure, high-density air-gas mixture. And to provide the containment needed for the pressure chamber to enclose the abyss.

If magic is used, there might be a simpler solution. Assume the magic works as a powerful viscosity amplifier. This will not only stop arrows in mid-flight, but enable people and rocks to float in its medium as everything would sink extremely slowly. Since this is magic. The viscosity will operate in such a way as to not prevent air flowing to any living creatures present in the abyss. Magic without safety procedures or mechanisms would be unnecessarily dangerous. It could also be designed to permit the movement of living creatures as if they were moving through a medium closer to normal air while preventing them from falling downwards.

A second simple magic solution would be a field of gravity neutralizing magic present in the abyss. Although this doesn't explain its arrow stopping properties. Perhaps that's a safety feature.

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