I have a large underground fantasy world which is only accessible through the Mariana Trench. I'd like to do a day/night cycle and have have a bright, sunny atmosphere part of the time, but I'm not sure how to get that much light since the "sky" is solid rock and deep underwater, and very high up from the bottom of the realm.

I'm thinking some sort of bioluminescent bacteria/algae, or light-refracting crystal, but I don't know if that would work. The waterways are full of bioluminescent plankton, and a lot of plants and animals have at least a little bioluminescence, so whatever is in the ceiling would have help, but that alone wouldn't illuminate it as much as I need.

This is a fantasy world, so things like electric lightbulbs and such aren't a possibility.

  • $\begingroup$ I'm assuming that by "fantasy world", you mean that it doesn't have modern technology? Or do you mean that things like light bulbs could never be invented? And are there alternate forms of energy, such as "magic"? $\endgroup$ – Laughing Vergil Jan 21 at 23:11
  • $\begingroup$ No, no modern technology in there. And magic does help with bits of it (Such as how there are little holes into the world from the ocean floor that let water in without breaking the roof or flooding the place) But i'm trying for a realistic light source to kind of even it out. $\endgroup$ – Guest Jan 21 at 23:42
  • $\begingroup$ I'm tempted to ask what you believe you'll gain from this. Sunlight levels of brightness call for a tremendous amount of energy. The price of having "realistic" light sources may be having tremendously unrealistic energy balances. Could they perhaps have lower levels of brightness, and have eyes adjusted to that/ $\endgroup$ – Cort Ammon Jan 22 at 0:56
  • $\begingroup$ Atmospheric appearance, mainly. It doesn't need to be quite as bright as actual sunlight, but I want it to be daytime light levels. No heat needs to be produced by it either. $\endgroup$ – Guest Jan 22 at 2:09

How to create a sunny atmosphere without an actual sun?

Marianas Trench.

What you have is one continent sized sheet of the Earth's crust being forced under extreme pressure beneath another incredibly massive sheet of rock.

enter image description here

GNU Free Documentation License

Like a balloon's surface being stroked with fur, but a billion times more powerful, electrical charges build up. On the surface they're known as Earthquake Lights when they discharge. Some static in nature, some piezoelectric, and here they are of such magnitude and over such an area more than all the power stations on the surface could generate, and they are buzzing for release.

With the pulse of nature's clock, this charge arcs across the gaps between subterranean mountains, filling natural reservoirs of copper and rare elements with its charge, and thin seams steadily conduct this charge to your caverns and phosphors in the roof eat-up this free energy, spitting it out in a thread, a stream of light upon your world through electroluminescence. Until the charge is exhausted and darkness falls - until nature's clock again proclaims its time, and the great charge breaks through with a rock-muffled thunderclap to bring day's light once again.

  • $\begingroup$ How long might one of these charges last? (How long could a day be?) And how long to recharge for the next day? (Length of nighttime?) $\endgroup$ – Guest Jan 22 at 14:07
  • $\begingroup$ @Guest That's the sort of thing that should be author's prerogative - ie, as long or as short as it needs to be for tou to tell your story most effectivley. Same with the light intensity and blend of spectrum you want. There's no reason that you couldn't have seasonal variations over periods of hundreds or even thousands of days either - it's up to you, it's your story. $\endgroup$ – 011358 smell Jan 22 at 14:36
  • 1
    $\begingroup$ It's worth noting that this is science-related handwavium - piezoelectricity isn't nearly sufficient to generate the intensity of light the OP is talking about, the kind of alloys required to "store" electrical charge do not occur in nature, and persuading phosphors to light up reliably is a non-trivial electrical engineering challenge. It's "sciency", but not "science-based". Kind of like Dan Brown's stuff. $\endgroup$ – jdunlop Jan 23 at 2:00
  • $\begingroup$ @jdunlop Absolutley, couldn't agree more. We could churn out half-nonsensicle figures, but science based is no, here. Sci-fi, yes, emphasis on fiction. $\endgroup$ – 011358 smell Jan 23 at 2:10

Radioluminescence can give you enough light, and not only to see, but provide an energy basis for life in these isolated conditions.

Please also note that if you want to make your story plausible, you need to find fixes for other issues, like enormous pressure at the bottom of Mariana Trench.

  • $\begingroup$ It's kind of under the Mariana trench, and there's enough rock in the way to help deal with that. The water falling out of holes in the roof is in small spots, and regulated by magic, but the light issue is the big thing. But yeah, radioluminescence seems plausible. The only thing I'm wondering about would be if it would be bright enough for what I want. I'm trying to get an atmosphere similar to our daylight without actual sunlight or electricity. $\endgroup$ – Guest Jan 21 at 23:44
  • $\begingroup$ How would the radioluminescence be "turned off" at night? I suppose magic could handle that. $\endgroup$ – BrettFromLA Jan 22 at 1:07

Since you labeled the question as "science-based", I have to warn you that no scheme you might implement will stand up to any scientific analysis:

Your cave can't exist

Simple as it is. The location you've chosen is 11km under the ground, so the rock is under extreme pressure. Under these pressures, even rock becomes plastic, so any cavern will collapse over time.

Also, even if your cavern does not collapse, it's still in a region where rock is being bent (because the pacific plate is forced under the Asian plate (rock that's plastic!)). So cracks will show pretty much wherever the rock is too hard to flow. And you've got a water pressure of more than 1.1 metric tons per square centimeter (1100 bar). This pressure will press the water into any sizable cavern.

Your light lacks an adequate energy source

The power of the sun is astounding. It provides us with 1.3 kW/m^2 when it's in zenit. Ok, let's say you settle for some softer illumination, say 100 W/m^2. A square kilometer has a million square meters, so for an area of 2.5km x 4km, you already need 1 GW of power. That's a nuclear power plant. Just for lighting an area of 2.5km x 4km to levels that don't even come close to what our sun provides.

Your cave lacks adequate cooling

Even if you managed to handwave the power source, you still have the problem of the power drain. You see, those Gigawatts need to go somewhere. And if you don't provide for a way for them to reach the outside, the energy just stays in form of heat. Did I mention that the rock is already typically quite hot at these depths? Great, now you are adding even more heat.

You cannot believably use sea water for the cooling, because, as pointed out above, any water near your cave spells doom to air breathing life within.


You may write an interesting story with your deep underground cavern. Yet this story will be more in the fantasy realm than in the (soft) science-fiction realm. Hard science-fiction cannot contain your brightly lit cavern.

  • $\begingroup$ I actually didn't label it science-based, someone else edited and put that in the tags. And it was supposed to be fantasy anyway, I just intended a little bit of science (i.e light sources) $\endgroup$ – Guest Jan 23 at 1:49

I'm going to use a massive scientific liberty here for a possible solution, but it is close enough to the truth to pass in science-fiction.

If the top of the world was covered with or made of Unobtanium, a fictional mineral capable of interacting with a large amount of the neutrinos passing through the earth at all times due to its magical properties. When neutrinos collide with matter (incredibly rare in real life, however Unobtanium could interact with say... 10% of passing neutrinos), they produce bursts of light.

Additionally this could be used as a fictional power source, with unobtanium being used to build neutrino collectors that power the city. Neutrinos contain a ton of energy, they just don't like interacting with regular matter all that much. If you really wanted to push the concept you could say that all the creatures underneath the trench have been exposed to Unobtaniums since birth and that could be a potential source of magic.


The top of the cave is filled with Neon gas. Neon gas is lighter than air, so it would float above the rest of the atmosphere of the cave. Neon emits bright light when electricity runs through it. That's how Neon lights are made, by putting neon gas into a glass tube and sending electricity through the gas. You could have the static electricity from Fay Suggers' great answer go through the upper atmosphere of your cave. The effect would be like there's a very bright glowing cloud across the top of the cave.

  • $\begingroup$ "Neon gas is heavier than air, so it would float above the rest of the atmosphere of the cave." No? $\endgroup$ – jdunlop Jan 23 at 1:55
  • $\begingroup$ chemistry.stackexchange.com/questions/61140/… the Neon would mostly mix, but only the top layer would have the electrical charge making the neon would illuminate more in the sky; so this may still a viable answer. $\endgroup$ – Nosajimiki Jan 23 at 3:06
  • $\begingroup$ @jdunlop oops meant to write 'lighter than air' I'm not sure how I managed to get it backwards. $\endgroup$ – Jared K Jan 23 at 16:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.