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  • In my world, there would be a tropical region very similar to real life rain forests, though underground there's still jungle. At a point underground, there are huge caverns that are populated by various tropical plants, trees, fungi, algae, along with other types flora and various fauna as well.

  • I'm aware that plants require photosynthesis to survive, though there's no access to sunlight. All light for photosynthesis would need to be from other sources, such as organisms with bio-luminescence bright enough to support decent light, fires, etc.

  • Is there any way a climate like that can be scientifically possible?

  • If you want any more details about this, I'll be happy to do so.

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  • $\begingroup$ You can write a markdown list by having an empty line and then starting a line with "- " (the space is important). That looks better than your self-made solution because it's consistent with the general site layout and how lists are presented on other questions and answers. In this case though I think the list was unnecessary and only distracts from what you are asking as you don't have any content that fits a list, so I removed it, but please feel free to roll back the edit or edit further if you prefer the post in list format. $\endgroup$ – Secespitus Jun 5 '18 at 16:04
  • $\begingroup$ Ok, thanks for letting me know, I'll make sure to use "- space"(hopefully that's right) to separate sentences in future questions. $\endgroup$ – BlueTangsRock Jun 5 '18 at 16:11
  • $\begingroup$ I think something similar was already asked in the past... $\endgroup$ – L.Dutch Jun 5 '18 at 16:11
  • $\begingroup$ @L.Dutch may I have a link to it if you find it, so I can view if their question has the same idea? I'll try to search to see if there's any possibile duplicates. $\endgroup$ – BlueTangsRock Jun 5 '18 at 16:14
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    $\begingroup$ Google "New York City waste treatment facilities." The answer is absolutely "yes." $\endgroup$ – SRM Jun 5 '18 at 17:58
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The problem with plants is that they need light, and they are inefficient (using only about 1% of available energy) so they need lots of light.

A star can provide that, lucky for us. But underground that light can't get in. Other light sources are orders of magnitude dimmer (e.g. biological or natural phosphorescence). You could have a very hot body emitting blackbody radiation (for example some sort of natural fission reactor, or very hot lava) but the problem then is heat. The air would get crazy hot and the plants would cook.

You need to wave your hands. Wave them now. Feel the breeze.

1: Natural LEDs. These creaures would harness some sort of natural current (maybe piezoelectric current generated by tectonic plate motion?) and put out visible light as a byproduct. There might be a limited spectrum which fortunately for your plants would include the red light they use.

2: Ancient tech. - Maybe the ancients set up lights down there for some reason. Maybe illumination was not even their intent. For example

2A: Portal. This huge portal to another place has a view of a star and admits light from that star.

2B: Super swanky alien tech lights powered by exotic energy source (e.g. zpoint energy, casimir forces etc). These machines actually are supposed to do something else, and the lights are just indicators. Do you want to see if you can make them do the something else?

2C: Gobliny steampunk tech lights powered by nonexotic energy source (e.g. geothermal electric generator). The goblins keep these running.

  1. Magic. Sure, magic you up some light.
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  • $\begingroup$ You forgot to add making the plants super efficient in their light usage. $\endgroup$ – Innovine Jun 7 '18 at 5:43
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Radium lighting or a similar radioactive decay based system might perhaps work. To keep the temperature in the tropical range you need to bury your cavern at least 750 metres to a kilometer down if it's completely sealed, deeper in accordance with how much it leaks heat, you can estimate based on the geothermal gradient how deep you need to be to get rock hot enough to maintain the necessary temperature.

Now for the big problem, water, the issue is not getting enough of it but not getting too much of it. "Nature abhors a vacuum" if you create a void that far below the water table it will, in the natural course of things, fill with hot mineral saturated water, the generic terms is hydrothermal fluid. I'm not sure if you can have the space hot enough to force water up and out in the form of vapour and have living organisms in it at the same time but maybe a balance can be struck.

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  • $\begingroup$ If the roof of the cavern is impermeable it could trap enough gas to maintain a pocket of air no different than natural gas or helium deposits. Of course how that particular mix of gasses got there in the first place, and the organisms living in the cave, is another matter. $\endgroup$ – pluckedkiwi Jun 5 '18 at 18:58
  • $\begingroup$ @pluckedkiwi Yeah maybe, if it's a completely sealed system like some monster terrarium, as soon as you punched into, or out of, it the results would be devastating. $\endgroup$ – Ash Jun 5 '18 at 19:01
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You could have some special plant which is on the surface, collecting light, and has long roots hanging down into the cave system, which work like fiber optics and dump waste light to the cave plants. Maybe some symbiotic benefit here, the jungle plants live on the light, exhaling co2 and moisture which the root system absorbs.

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  • $\begingroup$ I can see how the cave plants could benefit, but how could those trees benefit with plants underground growing on their roots? $\endgroup$ – BlueTangsRock Jul 15 '18 at 5:01
  • $\begingroup$ The underground plants could exhale gasses which the roots absorb, as stated in the answer above. $\endgroup$ – Innovine Jul 16 '18 at 14:09
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I don't think so, or at least not in a world without magic, the thing about the sun its how much energy it outputs, energy that its captured by plants, to my knowledge there are no comparable sources of energy underground other than geothermal heat and natural sources of radiation but either of those two thing happen way to deep for a cave to naturally form, so if by some random chance of one in a million where to give a deep cave this conditions it could be plausible to develop some sort of complex environment, its just that beings in there wouldn't be plant like, they would be most likely fungus as those thrive way better in those conditions

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You could presume some alternate sustenance than photosynthesis, and combine it with what creates "hot" caves here on earth - a geothermal vent in a large cavern, with non-photosynthetic, probably mostly fungal plant life converting CO2 from the vent into O2. Bio-luminescence can provide light, but not enough to allow for photosynthesis.

It would definitely be a stretch to try to and set this up "realistically."

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Is there any way a climate like that can be scientifically possible?

You need two things:

  1. A source of sunlight (and heat).

  2. A thermal sump capable of maintaining equilibrium, otherwise your pocket underground universe would find a thermal equilibrium with a very hot Sun-lookalike. Everybody gets boiled.

There is very little chance of a "natural" origin for either, so you can imagine the underworld was established by a technologically advanced race as, maybe, an alternative to full terraformation (Spinneret by Timothy Zahn features such an environment). Then the occupants moved away, and the world ran amok, finding an equilibrium in a much more jungley state than before.

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