Planetary Geology of Athas

Continuing the discussion from Suggestions for Athas Underdark mobs?:

Hey everyone. Something came up today on the Underdark mobs discussion about the water table. And it occurred to me that it might be worth geeking out a bit on the actual geology of the planet. Since I’m thinking about a medium term project now which would be affected by this, I thought I’d ask what everyone knows (canon or otherwise regarding):

  • Athas’ gravity (explaining the size of the monsters)
  • What’s going on in the inside of the planet
  • Is there still water on Athas? Where is it, where does it come from, how deep do you have to go to find aquifers, are there underground rivers/lakes, etc.
  • Anything about the side of the planet opposite the Tablelands

Now I expect this is going to be external source/speculation/head-canon/bullsh***ing heavy, so purists might as well just look away now…

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I’ll take a stab at this

Gravity: I think the gravity on Athas is approximately the same as what we have on Earth. Though the rule books don’t specify anything on this, they don’t mention any change to it. The rules for falling, DCs for climbing and such remain all the same. 9.8 m/s^2. I think you can use an appropriately earth sized planet and mass. Perhaps the size of monster has more to do with the amount of oxygen in the atmosphere.

Inside: I would assume Athas has an iron core similar to earth. Lots of volcanic activity.

Water: not much surface water. Maybe underground water deep deep down or maybe below some areas of the sea of silt. Geysers could be a source of water at the surface. Like Yellowstone. But im no geologist so how it gets down there im not sure…. It’s possible there is some kind of underground lakes or water filled caverns

Opposite side: for the DM to determine.


My understanding (from looking at way too much science stuff in general) is that water gets stuck underground as tectonic plates under the ocean shift and one drives under the other.


Gravity on Athas would be significantly less than Earth because of the lack of metal, unless most of the remaining metal is still at the core of the planet. Most of the planets in our solar system are comprised of a molten nickel/Iron core with residual heavy and noble metal only coming to the surface through volcanic eruptions. It seems that there are still some areas on Athas that still have active super volcanos, that spew out much of the metal from the core, and all the obsidian obviously points to catastrophic eruptions that evaporated large bodies of water. So the likelihood of the Athas core having a significant molten metal core is diminished… I assume that the gravity on Athas is anywhere from 40-80% of the gravity of earth, but probably more in line with the gravity of Mars which is about 40%. But only travelers from other planets or the 2 moons would feel the difference.

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This is why someone made an outlander character kit for John Carter fans, giving outlanders a +2 strength.

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From the other forum: Suggestions for Athas Underdark mobs? - #21 by Jason_Goff

I’ll chime in from a rules point of view:

Prime Material Planes all have earth standard gravity and time flow unless specified otherwise in the rules.

The earth standard rules of physics are implied to be the default setting for Primes in the PHB where it occasionally comments about breaking said laws with magic. For the record, psionics also break the laws of physics.

Psionics and magic freely override the laws of physics wherever in operation (ie: Land of Black Ice, and the overall climate in the Flaaness in general, by way of example).

Both technological starships and spelljammers operate in the same Prime (Greyhawk, thus by default all the rest too) but perceived the universe differently when traveling, which implies some interesting things about planar topography and interactions.

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I would keep Gravity as earth, but I am sure there are anomalous areas out there (ie like wild magic). As for the water table, Id say it varies depending on the landscape above.

The Oxygen of the planet could be upwards of 30% to somewhat explain the large creatures. But ten again we don’t change it in other worlds so might have to chalk it up to just fantasy.

As for the metal-poor world I recommend this: science based - Physics of a metal-poor world - Worldbuilding Stack Exchange

But as said before Magic and Psionics probably forces us to ignore many things that would be prevented if we applied actual science to it.

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I always figured Athasian gravity would be lower than Earth simply to allow for the existence of the biggest silt horrors, Athasian giants, and other similar things.

But it’s interesting to think that higher oxygen could compensate for higher gravity to allow for the existence of bigger creatures.

I’m not a fan of relying on magic to compensate for the laws of physics, but there’s no other way to make some things work I suppose.

I suppose this also opens us up to the question: was Athas always metal poor, or was it a side-effect of the world becoming depleted?

We do know that Athas isn’t completely absent of metals. It’s just not that common. Tyr does have an iron mine.and book talk about the use and work of it being lost.

In the Wanderer’s Journal, page 9, from the original boxed, set he talks about metals being rare, but he also surmises that it hadn’t always been that way.

“As I have stated earlier, it is my belief that metal was not always scarce on Athas. For the last few centuries, our main source has been debris from the ruins of ancient castles and cities. Apparently, our ancestors devoured Athas’ ore supply, leaving to us little but their scrap. Now, even that meager supply is all but exhausted, and with it fades the ghost of civilization. There are those who say that our cultures and technologies can survive without metal, but I believe that they are wrong.”

Perhaps the depletion is localized. It’s also possible that the location of the city-states are just not in a metal rich location close to the surface. Somewhere in another thread or discussion there were estimates that the campaign setting takes place in an area about the size of Colorado. If that is the case it’s reasonable that other areas on the planet may have iron deposits. Or it at least suggests the planet has an iron core like Earth and gravity can still be the same.

For monster size, I think it’s reasonable to look at Earth’s past environmental conditions. Compare these to dinosaurs and their size for terrestrial creatures and also look at ocean creature size like whales for silt dwelling creatures.

I’m not sure that an increased amount of oxygen on a planet would have changes to gameplay rules that lowering the gravity would. (A can of worms in my opinion)

I too am not a fan of relying on magic compensate for the laws of physics. I like having a solid foundation we are all familiar with, that can be relied on, and described by physics and then magic and Psionics breaks those rules in ways physics can’t describe.

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The Rules book states on two occasions that Athas is not metal-depleted but metal-poor. I think thats where most of us get the theme. Although in the WG how he talks about the ruins and metal it does seem to indicate that Athas is metal depleted, as well as his statement.

Personally I think both can be true.

Athas is metal-poor (due to the changes wrought by Defiling, mislaigned elemental planes have something to do with it.) and Metal-depleted (what was available has been used up during the last two millennia).

So basically Athas was normal until the ravages of defiling changed the nature of the land which permeates deep into the earth. As the civilizations fought in the world-shattering wars they used the mines available to the fullest extent until all known sites were metal-depleted.

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I could live with that. The existence of gems and minerals/metals are often depicted as being boons from Elementally-aligned powers/gods. It’s not a long stretch to suggest that as the power of the Earth Elemental lords waned, the bounties of the Earth dried up as well (a similar process happen with Elemental Water after all); its true of the soil, why not metals?

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The vast plains of Obsidian says to me that the core of the planet is mostly silica and very low in metal, silica and water is needed to make obsidian, which makes sense considering that the entire planet is covered in a silt and silica sea. It had to come from somewhere, and it didn’t come from Space or the Astral plane.

The vast plain of obsidian that dumped through a magical gate from the Elemental Planes?

If that’s how you want to run it, that is fine, but I thought you were one of those people that don’t believe in extra planar gates, seems like a contradiction to me.

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Generally you’re right. Absolutely.

Specifically in the Deadlands, there IS a magical gate and it DID puke up the obsidian. It’s the central plot point of the region.

It’s a stupid McGuffin-y thing, but I didn’t invent it and its too tied to everything else to eliminate. The idea’s been on the books for ~30 years, but its pretty obscure relative to other setting details.

I suppose it was unfair of me to assume you’d (or anyone, really) known about it. Maybe you had, maybe not - whichever. Its cool. :grinning:


While I’m also not a fan of using magic as just a waveoff explanation for the apparent breaking of the laws of physics, it is important to remember that if there’s an available source of energy or resource, life will evolve to take advantage that.
On earth, there are animals who can navigate by sensing earth’s magnetic fields, or hunt by sensing electrical signals of other living beings.
It isn’t a huge leap in logic to assume that on athas, where it is already established that everything is psionic, that animals would use these powers to defy the natural laws of their harsh environments to survive. (there was an excellent netbook on the subject somewhere)

To surmise, so long as you elaborate, however vaguely, how a being would use supernatural forces to seamingly break the laws of nature, magic can be a viable explanation for why things the way they are on Athas and not on earth.

Remember what Jeff Goldblum taught us: “Life… eh… find a way…”

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I read somewhere that some people suggest that Athas is a young planet in regards to the formation of metal ores,
in fact seems that sorcerer monarchs hide the fact that metal is deep into the mountains and the earth
Also the wars (cleansing wars wipe entire races) and the ecological cataclysms seems to used up the remaining metal easy to get, notice that easy to get iron are either in swamps or near water filled areas

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The thing here is that we can’t take the Geology of Athas too seriously in the context of a real planet for a few reasons.

Just the changing of the Sun from Blue to Yellow to Red is tough to explain as the goldilocks zone, radiation levels and world temps would vary drastically. Looking at real Astrophysics and Exoplanetary Science makes many of the past occurrences in the history of Athas tough to explain without some metaphysical explanations to account.

I believe the goal would be to make Athas makes sense from a geologic perspectives while also acknowledging magical/extraplanar influences. We all know many of the map feature from the original map to the expanded official maps don’t make much sense.

There wouldnt be a Sea of Silt thousands of feet thick if the oceans actually boiled away to high temps we would just have huge dusty basins similar to what Mars actually has. And yeah the Obsidian unleashed by Quith came from somewhere. (my view is she opened a portal to the Para-Elemental Plane of Magma and the Negative Plane (The Black for some) and when things cooled rapidly all that was left was obsidian.

In a place where Arcane, Divine and Psionic powers are real we can bend 'reality ’ to come up with some reasonable assumptions.


The sun going from blue to yellow could be due to siphoning off of the energy by a black hole( or the grey or portal to the Abyss even…) or another celestial body like another sun that is close by. There would be an intermittent phase where it would be green though, which actually could be the real reason why it is called the Green age.

If a blue sun consistently loses its plasma to another celestial body (or through magical means) it will not supernova, but will cool down to a lesser wavelength. The sun in the Dark Sun age would have to be a red giant to put out the amount of heat and radiation that the planet is currently dealing with though, which would mean it would have to have expanded so that it is closer to the planet. Either that or the orbit of the planet was changed at some point and brought closer in to the star. Red Giants typically collapse into white dwarfs or quasars, but they can sometimes supernova if the right conditions are met.

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