There is nothing wrong with having the world lit by the internal core. It is just day all the time, or like the Titan worlds you can have plates that move about giving "night" periods.
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For me a "hollow earth" only makes sense "because magic".
Or technology. If going for a hollow earth, I would premise an advanced civilization created the whole planet artificially, by means of magic or technology.
Personally, I prefer the creatures living inside the stone, being part of the stone, phasing thru it, such as a civilization of earth elementals who move thru stone (earthwalk), or perhaps an entire ecosystem of living beings. (One can do the same with magma fire elementals.)
In an animistic world, the stones are conscious beings, and one can enter the dreamscape within the features of stones, mentally, in an outofbody spirit journey. The minds of stones are nonhuman, but can project into the human world as if human, similar to the way human shamans can take the form of animals or stones.
I haven’t read the thread to check, but just in case this hasn’t been mentioned, in the 1e/2e official world of Mystara was hollow, and that Hollow World was its own setting.
Mystara is a hollow planet, with a habitable surface on its interior called the Hollow World. This world is lit by an eternal red sun at the center of Mystara, and serves as a "cultural museum," preserving the societies that have become extinct in the outer world. The existence of the Hollow World is not, in general, known to the inhabitants of the outer world. The poles are actually huge, subtly curving holes that allow passage between the outer and inner world, although it is a long, hard trek through a cold, unlit, stormy and anti-magic area. The curvature of the holes is so subtle that explorers from either surface do not notice the transition until after it is already made, causing quite a shock for most.
Another alternative would be to research the D'ni civilization from the Myst franchise.
You need daylight-level light, everywhere, underground, without any geothermal source, chemical (bioluminescent/phosphorescent) source, or magical source. . .. ... ...nope. Nothing doing. "Underdark" is dark for a reason - light doesn't penetrate the ground well. if you want to set up thousands of mirrored shafts all throughout your world to bounce sunlight down through several miles of rock and soil, and somehow set these shafts up to not be broken by tectonic activity or result in the whole underground flooding out the immediate moment any heavy rains hit the surface? Be my guest. But after reading this thread I think you're asking for way too much, man.
It's okay to handwave something sometimes. Really. Use luminescent crystals, or rare moss with exceptionally bright bioluminescent properties. Only way you get to have daylight-level light underground without straight-up magic. Or, y'know, electrical power grids and streetlights.
You see, you claim that Bioluminescence isn’t strong enough, but the reason I suggested looking into Myst was because they explained how it did work in that worlds lore and for scientific reasons. Their civilization was built in a cavern that held a massive underground lake, and their civilization was built on the shores of the lake, and the walls of the cavern. That lake contained a bioluminescent algae (or bacteria or something), and the sheer quantity and concentration of algae (or bacteria) could fill the entire cavern with at least the light equivalent of D&D dim light. And the colony of bacteria (or algae) had its own biorhythms and during the colony’s sleep phase it would go very dim to provide a sort of natural night time. So, scientifically plausible to say the least.
Mystara’s Hollow world did have Magic, but it was very wonky because of the Magic that the Immortals (Mystara doesn’t have gods) used to keep the Hollow World working. So, with your world not relying on much Magic, this could dovetail nicely. However, if you don’t want divine forces doing the thing, then you could still use science to fuel your DM’s hanwavium by conceptualizing it as a miniature Dyson Sphere
Just hand wave it and say there is. Or let the people without dark vision have a disadvantage and carry lots of torches or have access to the light spell. This kind of remind me of the threads that pop up where a person wants to have a character that’s blind, but doesn’t have any of the drawbacks of being blind. What’s the point of the exercise if it’s a hollow world, but there‘s ample natural light just like in a more traditional world? Either go with the magic or other options people have presented here, which in some ways (to me at least if you’re trying to do it on a global level) kind of defeats the purpose of having this unusual world, or you lean into it and tell the players, yeah, it’s pitch black, figure out how to deal with it.
To the OP, I would strongly recommending revisiting one of your initial assumptions. Unless they have only recently fled below, all permanent residents of any "hollow earth" are extremely likely to be capable of operating in low or no light.
If you have any existing ecology below the surface, any topsiders that fled underground without sufficient magic or tech would have been eaten or got lost etc. Anything that has evolved to live below would certainly have some form of dark-sight, Darkvision, echo location etc.
A balance between your assumption here and the debate from other posters on the strength of bioluminescence or magma light is to give creatures some kind of partial Darkvision, e.g. the dim light bit, but not the ability to see in full darkness perhaps?
If you want there to be light because your adventurers are surface dwellers without Darkvision and they are going underground, then there's a whole adventures worth of content there first for them to discover a magic item, potion, special plant etc that will allow them to see while they are below. A consumable would be neat as this potential puts them on a time limit for their business underground.
If your world is a true hollow world - basically a spherical outer shell - and you have things living on the inside of the shell - you've already hand-waived away gravity - so hand-waiving a light source shouldn't be a big deal.
If anyone mention this I apologise for being to lazy to read everything....
Sounds like OP needs to have a look at the Blackreach area of Skyrim, its very "underdarky" so gives a good way to show of the underground lighting so could suit the hollow earth vibe......
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Who says the creatures/plants have evolved to need Vitamin D? And you can have a variety of light sources, such as veins of crystal that shine with surface light in some parts, bioluminescent light in others, magma in others and chemical in yet others. Imagine big glowsticks. And many creatures might navigate more by echolocation than darkvision.
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It did seem odd to me that OP was worried about the requirement for creatures to have the effects of sunlight in a world where they evolved without it...
For a more industrial theme, you could have great metal wheels sunk into the magma which are forever slowly turning, drawing the white-hot parts of the wheel up to light and heat the cavern and sinking the now cooled and dim parts of the wheel back into the magma. Alternatively a giant piston system which lifts glowing metal out and sinks cooled metal back in.
Bioluminescence is one way, but you've already expressed dislike for that idea. What about rudimentary electrics? It stands to reason that in a populated, perpetually dark place, the search for a light source which doesn't involve digging up to the sun or burning precious oxygen could have led to basic electrical breakthroughs? Without magnets there's no risk of motors and other advancements, but basic lightbulbs could be an option. Or a crystal which glows when it is presented with electricity. The system could be powered by giant piezoelectric crystals which are charged by the shifting of the tectonic plates.
Another option would be to have torches made of a type of wood which oxidises itself, so doesn't consume oxygen. It would probably have to be produced specially, so would probably be supplied by a massive company which could have all kinds of nefarious goals - or might need help sourcing more of the wood - perhaps the wood they use is dying now, and they need the party to find out why - it turns out the roots were for trees which have now been felled. The company down below might think it is some disease slowly spreading through the roots, but then find out it is due to the trees being felled systematically to clear space for a village, or something like that.
Or, if you don't want to use magic for safe, non-flammable light; what about electricity? How advanced is your subterranean society? They must be fairly advanced to dig all those tunnels.
I think folks should clarify that a "hollow earth" is not necessarily the under dark. An Underdark is a fantastical location ecologically and geologically speaking but still predicated that you're living beneath a world that sunlight doesn't reach. Some Hollow Earths follow that model, but from what I'm gathering the OP is looking for the Hollow Earth in line with (IRL debunked) thinking that there's a world beneath the surface that has surface world level lighting and consequently surface level flora and fauna inhabiting it (hollow earth lore is big in cryptozoology saying creatures like dinosaurs etc survived their existential cataclysms by falling into holes leading to it). So I believe, given the OP's comment about Vitamin D etc. the OP wants something akin to Jules Verne's Journey to the Center of the Earth. I don't remember how Verne's hollow was lit, and if I recall it wasn't perfectly lit but perpetually cloudy and often stormy); but doing a wikipedia lore refresh, a neat idea would be Edmond Halley's 17th century theory which posited an interior earth with a luminous interior which occasionally leaks to the surface causing the Aurora Borealis (cool, and kind of a Spelljammer phlogiston tie in, maybe a bit less volatile). The "gravity" objections are sorta hand waved with the interior world either being a sphere within a sphere and or the interior having a separate magnetic pole system.
Another thought would be rather than adjacent demiplanes, the Feywild and Shadowfell are actually contained within the game world's interior (and wax poetic on literally making expressions of the extremes vibrancies and darknesses of existence not just be something within the interiority of the individual but the interiority of the world itself ... "deep" so to speak). So instead of planar shifting or summoning from Feywild or Shadowfell, denizens and visitors just go on really really long walks or climbs.
I will say I find a lot of the more underdarky brainstorming really inspired and could see a world that has a "surface" and under dark and one or two interiors (maybe Vernesian hollow earth, with the brightness and darkness at an even deeper core).
I still like the world core as inner sun, with attendant need for sunblock for underdarkers who delve that deep.
There is nothing wrong with having the world lit by the internal core. It is just day all the time, or like the Titan worlds you can have plates that move about giving "night" periods.
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You can have an "inner moon" like Pellucidar. In that series, the moon didn't move, but I'm not sure why it couldn't.
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For me a "hollow earth" only makes sense "because magic".
Or technology. If going for a hollow earth, I would premise an advanced civilization created the whole planet artificially, by means of magic or technology.
he / him
Personally, I prefer the creatures living inside the stone, being part of the stone, phasing thru it, such as a civilization of earth elementals who move thru stone (earthwalk), or perhaps an entire ecosystem of living beings. (One can do the same with magma fire elementals.)
In an animistic world, the stones are conscious beings, and one can enter the dreamscape within the features of stones, mentally, in an outofbody spirit journey. The minds of stones are nonhuman, but can project into the human world as if human, similar to the way human shamans can take the form of animals or stones.
he / him
I haven’t read the thread to check, but just in case this hasn’t been mentioned, in the 1e/2e official world of Mystara was hollow, and that Hollow World was its own setting.
Mystara is a hollow planet, with a habitable surface on its interior called the Hollow World. This world is lit by an eternal red sun at the center of Mystara, and serves as a "cultural museum," preserving the societies that have become extinct in the outer world. The existence of the Hollow World is not, in general, known to the inhabitants of the outer world. The poles are actually huge, subtly curving holes that allow passage between the outer and inner world, although it is a long, hard trek through a cold, unlit, stormy and anti-magic area. The curvature of the holes is so subtle that explorers from either surface do not notice the transition until after it is already made, causing quite a shock for most.
Another alternative would be to research the D'ni civilization from the Myst franchise.
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Related to the mirrors idea, you could have a chain of crystal mountains that sunlight passes through like fiber-optic cables into the hollow core
You need daylight-level light, everywhere, underground, without any geothermal source, chemical (bioluminescent/phosphorescent) source, or magical source.
.
..
...
...nope. Nothing doing. "Underdark" is dark for a reason - light doesn't penetrate the ground well. if you want to set up thousands of mirrored shafts all throughout your world to bounce sunlight down through several miles of rock and soil, and somehow set these shafts up to not be broken by tectonic activity or result in the whole underground flooding out the immediate moment any heavy rains hit the surface? Be my guest. But after reading this thread I think you're asking for way too much, man.
It's okay to handwave something sometimes. Really. Use luminescent crystals, or rare moss with exceptionally bright bioluminescent properties. Only way you get to have daylight-level light underground without straight-up magic. Or, y'know, electrical power grids and streetlights.
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Skfma,
You see, you claim that Bioluminescence isn’t strong enough, but the reason I suggested looking into Myst was because they explained how it did work in that worlds lore and for scientific reasons. Their civilization was built in a cavern that held a massive underground lake, and their civilization was built on the shores of the lake, and the walls of the cavern. That lake contained a bioluminescent algae (or bacteria or something), and the sheer quantity and concentration of algae (or bacteria) could fill the entire cavern with at least the light equivalent of D&D dim light. And the colony of bacteria (or algae) had its own biorhythms and during the colony’s sleep phase it would go very dim to provide a sort of natural night time. So, scientifically plausible to say the least.
Mystara’s Hollow world did have Magic, but it was very wonky because of the Magic that the Immortals (Mystara doesn’t have gods) used to keep the Hollow World working. So, with your world not relying on much Magic, this could dovetail nicely. However, if you don’t want divine forces doing the thing, then you could still use science to fuel your DM’s hanwavium by conceptualizing it as a miniature Dyson Sphere
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Just hand wave it and say there is. Or let the people without dark vision have a disadvantage and carry lots of torches or have access to the light spell.
This kind of remind me of the threads that pop up where a person wants to have a character that’s blind, but doesn’t have any of the drawbacks of being blind. What’s the point of the exercise if it’s a hollow world, but there‘s ample natural light just like in a more traditional world? Either go with the magic or other options people have presented here, which in some ways (to me at least if you’re trying to do it on a global level) kind of defeats the purpose of having this unusual world, or you lean into it and tell the players, yeah, it’s pitch black, figure out how to deal with it.
To the OP, I would strongly recommending revisiting one of your initial assumptions. Unless they have only recently fled below, all permanent residents of any "hollow earth" are extremely likely to be capable of operating in low or no light.
If you have any existing ecology below the surface, any topsiders that fled underground without sufficient magic or tech would have been eaten or got lost etc. Anything that has evolved to live below would certainly have some form of dark-sight, Darkvision, echo location etc.
A balance between your assumption here and the debate from other posters on the strength of bioluminescence or magma light is to give creatures some kind of partial Darkvision, e.g. the dim light bit, but not the ability to see in full darkness perhaps?
If you want there to be light because your adventurers are surface dwellers without Darkvision and they are going underground, then there's a whole adventures worth of content there first for them to discover a magic item, potion, special plant etc that will allow them to see while they are below. A consumable would be neat as this potential puts them on a time limit for their business underground.
If your world is a true hollow world - basically a spherical outer shell - and you have things living on the inside of the shell - you've already hand-waived away gravity - so hand-waiving a light source shouldn't be a big deal.
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If anyone mention this I apologise for being to lazy to read everything....
Sounds like OP needs to have a look at the Blackreach area of Skyrim, its very "underdarky" so gives a good way to show of the underground lighting so could suit the hollow earth vibe......
Who says the creatures/plants have evolved to need Vitamin D? And you can have a variety of light sources, such as veins of crystal that shine with surface light in some parts, bioluminescent light in others, magma in others and chemical in yet others. Imagine big glowsticks. And many creatures might navigate more by echolocation than darkvision.
Maybe glowing crystals?
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It did seem odd to me that OP was worried about the requirement for creatures to have the effects of sunlight in a world where they evolved without it...
Just make a radioactive rock...but instead of radiation, it just emits photons.
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For a more industrial theme, you could have great metal wheels sunk into the magma which are forever slowly turning, drawing the white-hot parts of the wheel up to light and heat the cavern and sinking the now cooled and dim parts of the wheel back into the magma. Alternatively a giant piston system which lifts glowing metal out and sinks cooled metal back in.
Bioluminescence is one way, but you've already expressed dislike for that idea. What about rudimentary electrics? It stands to reason that in a populated, perpetually dark place, the search for a light source which doesn't involve digging up to the sun or burning precious oxygen could have led to basic electrical breakthroughs? Without magnets there's no risk of motors and other advancements, but basic lightbulbs could be an option. Or a crystal which glows when it is presented with electricity. The system could be powered by giant piezoelectric crystals which are charged by the shifting of the tectonic plates.
Another option would be to have torches made of a type of wood which oxidises itself, so doesn't consume oxygen. It would probably have to be produced specially, so would probably be supplied by a massive company which could have all kinds of nefarious goals - or might need help sourcing more of the wood - perhaps the wood they use is dying now, and they need the party to find out why - it turns out the roots were for trees which have now been felled. The company down below might think it is some disease slowly spreading through the roots, but then find out it is due to the trees being felled systematically to clear space for a village, or something like that.
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What's wrong with fire? Lanterns? Candles?
Or, if you don't want to use magic for safe, non-flammable light; what about electricity? How advanced is your subterranean society? They must be fairly advanced to dig all those tunnels.
I think folks should clarify that a "hollow earth" is not necessarily the under dark. An Underdark is a fantastical location ecologically and geologically speaking but still predicated that you're living beneath a world that sunlight doesn't reach. Some Hollow Earths follow that model, but from what I'm gathering the OP is looking for the Hollow Earth in line with (IRL debunked) thinking that there's a world beneath the surface that has surface world level lighting and consequently surface level flora and fauna inhabiting it (hollow earth lore is big in cryptozoology saying creatures like dinosaurs etc survived their existential cataclysms by falling into holes leading to it). So I believe, given the OP's comment about Vitamin D etc. the OP wants something akin to Jules Verne's Journey to the Center of the Earth. I don't remember how Verne's hollow was lit, and if I recall it wasn't perfectly lit but perpetually cloudy and often stormy); but doing a wikipedia lore refresh, a neat idea would be Edmond Halley's 17th century theory which posited an interior earth with a luminous interior which occasionally leaks to the surface causing the Aurora Borealis (cool, and kind of a Spelljammer phlogiston tie in, maybe a bit less volatile). The "gravity" objections are sorta hand waved with the interior world either being a sphere within a sphere and or the interior having a separate magnetic pole system.
Another thought would be rather than adjacent demiplanes, the Feywild and Shadowfell are actually contained within the game world's interior (and wax poetic on literally making expressions of the extremes vibrancies and darknesses of existence not just be something within the interiority of the individual but the interiority of the world itself ... "deep" so to speak). So instead of planar shifting or summoning from Feywild or Shadowfell, denizens and visitors just go on really really long walks or climbs.
I will say I find a lot of the more underdarky brainstorming really inspired and could see a world that has a "surface" and under dark and one or two interiors (maybe Vernesian hollow earth, with the brightness and darkness at an even deeper core).
I still like the world core as inner sun, with attendant need for sunblock for underdarkers who delve that deep.
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