I have this hollow mountain that has a closed ecosystem inside it. What's interesting is that vegetation thrives within the bowels of the mountain despite being almost completely closed off from the outside world. This is made possible thanks to minerals lining the walls that carry light inside via reflection. Like the mirrors of periscopes. The summit of the mountain has an opening for light to enter which is then reflected into a series of branching tunnels and into chambers. These chambers are safe from weather conditions but still receive light, rain water and airflow. Assuming there are no opaque walls midway of a tunnel the light could go very deep.

What naturally occurring minerals might line the cave walls to allow it to be illuminated this way?

  • 3
    $\begingroup$ Something to consider is that even if you could spread the light to fill an adequately large area, the mountain is only getting as much light as can fit into the hole. So if you have a 50m^2 opening and spread it out over 5000m^2, each part of your caves will only get an average of 1% as much light as the surface... enough to see by, perhaps, but not enough to feed thriving vegetation. $\endgroup$
    – Nosajimiki
    Commented Nov 4, 2021 at 22:08
  • $\begingroup$ /vegetation thrives within the bowels/ you had my upvote at this point. $\endgroup$
    – Willk
    Commented Nov 4, 2021 at 22:20
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    $\begingroup$ Willk's right, quartz is your best bet, but @Nosajimiki brings up a good point. You might want to consider something like lining the bottom of a large lake with your crystal deposits. Suspension-of-disbelief would allow people to believe that the sunlight striking the water was being refracted to the crystals lining the otherwise rocky bottom. It would allow you to capture a ton of light while keeping the "light entrance" hidden. $\endgroup$
    – JBH
    Commented Nov 5, 2021 at 6:09
  • $\begingroup$ Make the whole top of the mountain of Quartz. That should allow a nice large catchment region, and is not too geologically improbable. $\endgroup$
    – PcMan
    Commented Nov 5, 2021 at 11:05

1 Answer 1



quartz lamp


It is common. It is clear. It is tough and won't melt in the rain like gypsum might. You can make mountains of it. It comes in long needles which would look cool.

I like also the idea that the quartz crystals are piezoelectric and so in addition to lighting there is lightning inside the mountains bowels. I humbly request that as part of building your world you write an epic country song around that concept. It can be the theme song they play during the opening credits. I will get working with the steel guitar solo.

  • $\begingroup$ Looks nice that one. Now, pack its sides with with mud and see how much of the light still emerge from the top. Hint: tubular skylights/sun tunnels have a reflective inner surface. You may want to additionally handwave in your answer some gold deposits in that quartz to act as the reflector, especially in areas you need to seriously bend the light. (Re: total reflection - not gonna work, unless you isolate the entire quart column by a way less optical dense medium, like air. Then you'll have to deal with the trouble of water on those surfaces, especially condensation droplets) $\endgroup$ Commented Nov 5, 2021 at 0:26
  • $\begingroup$ @AdrianColomitchi, Real Life cannot be an overriding limitation on any question unless specifically requested. $\endgroup$
    – JBH
    Commented Nov 5, 2021 at 6:20
  • $\begingroup$ @JoinJBHonCodidact does it apply to comments made in answers context? I mean, I have this hobby to bring Real Life into the picture even when not specifically asked, is it forbidden? $\endgroup$ Commented Nov 5, 2021 at 6:57
  • $\begingroup$ @AdrianColomitchi Yes, frankly. This site has become so if-it-can't-be-done-in-real-life-it-shouldn't-be-done that it's sucking the joy out of worldbuilding. Will gave a great answer to a fictional scenario to create an effect the OP was looking for. Please learn from his example. $\endgroup$
    – JBH
    Commented Nov 5, 2021 at 7:06
  • $\begingroup$ @JoinJBHonCodidact Ok, time for me to leave WB alone then. Enjoy banking relativistic neutrons around the Moon's gravity well and popping up rules from Meta. $\endgroup$ Commented Nov 5, 2021 at 7:39

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