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Acording to my understanding of the word "swamp", we must have at least 3 components put together:

  1. Water in abundance
  2. Vegetation
  3. Soil for the vegetation to take hold (and eventually to produce mud when mixed with water)

So, I wanted to have a swamp, placed on an underground gallery accessible through a cave. This gallery should be very wide and high, but I'll not specify its dimensions (they should be defined in order to make the swamp plausible, not the other way around). What I want is for this gallery to not have an open roof, but to have a rocky roof that seals it almost completely from the exterior. Some fissures and cracks that allow some sunlight and rain to enter are permissible, but not so as to destroy the dark and gloomy atmosphere I'm trying to set here.


Given this, is it possible to build a swamp with the three components I mentioned?

  1. High water content is not a problem, since there are many instances of water in caves, sometimes even entire lakes and underground rivers

  2. Vegetation may be more problematic, since plants need light to develop. They could gather in the ground around the fissures and cracks I mentioned above, but not in a quantity that would produce a swamp. I don't need trees or other kinds of complex plants. Islands of moss floating about or crawling on the cave walls would suffice. Algae, lichens and fungi are also permissible.

  3. Soil is also problematic. Firstly, because hummus comes from degradation of dead biologic material, which pressuposes the aforementioned vegetation. Also, soil may also be produced through erosion, but the only erosion here would come from the water, since the cave interior would not be subjected to winds. (Maybe the soil could be washed into the cave through the entrance in the context of periodic inundations outside. Or it could be carried from a surface river that dives underground. Or maybe bat guano could acount for the biodegradation needed. These are just ideas I'm throwing out here)

Given these hurdles, is it possible to create a plausible underground swamp with the specifications I mentioned?

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Start with something like this Cave of Swallows Or at least something like it. Maybe make it bigger.Hang Son Doong is another point to draw inspiration.

We know we can get water under ground and you can make it move as fast or as slow as you like.

The need for vegetation gets started by the already abundant plant life existing in the cave.

Soil can be brought in slowly by the underground river. As an additional mechanism, if this is in or near a volcanic range a stable media for growth is already right there.

Here is how it gets closed in. an earthquake or landslide, maybe even a lava flow covers over the hole at the top. maybe a complete cover, or maybe a partial cover with lots of little holes to allow in some light. Just enough for your adventurers to be able to navigate conveniently by. Those kinds of small light wells can give you some lichen growth or maybe some moss that will grow, die off, and fall into the lower pit adding to the swampy soil underneath.

Underneath, have the fungi, mushrooms, and other such take over. The nutrient rich soil will help feed them, and as they die, they will feed the next generations of fungi.

You could even have a system of these caves in a chain as they are formed as sinkholes. Some, higher up, are still open to the sky. The water flows out, carrying vegetation and such to the next gallery, which is a closed off hole.

You have the possibility of a couple of different underground ecosystems here just by tweaking a variable here and there.

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  • $\begingroup$ Thank you for this answer. Also +1 for the idea of the chain of caves that would flow vegetation and soil to lower sinkholes. $\endgroup$ – Pedro Gabriel Aug 11 '17 at 18:04
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I'll politely disagree with your definition of "swamp."

  • Water, yes. Shallow water, specifically.

  • Decomposing biological matter, yes. Vegetation? Not necessarily.

  • Soil? well if that included lots of decomposing biological matter, then I've already agreed to that.

Fiction is stuffed full of "dead" swamps - meaning live vegetation is not necessary. You need some energy source, and we know of several other than light. (Mostly in deep sea vents, and other hydrothermal environments).

I could easily imagine an ecology based on, say, a hydrogen sulfide metabolism. It could have plants, bacteria, and maybe even multicellular organisms (amoeba). The problem with this would be that H2S is lethal to us humans save at very low levels. So, the only way a person could survive would be to avoid the core areas of this environment OR there would have to be fairly steady winds ... or both.

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  • $\begingroup$ This is a good answer, but unfortunately, being completely lethal to humans would be prohibitive for my plot. However, maybe an alternative energy source could be imagined... volcanic energy in the form of hot springs? Some bacteria could survive on that... $\endgroup$ – Pedro Gabriel Aug 11 '17 at 18:06
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The difficult one is your second point.

Water should be easy, plenty of caves have water in them. Associated with the water, mud is pretty likely, (I wouldn't however necessarily call it soil without a lot of plant material)

As for the plants, you want a dark gloomy atmosphere, plants generally don't grow without light, so you might get some growth in fissures or sections of the cave open to the light (like sinkholes), but your best shot for swamp plant-life is fungi, but they require decomposing plant matter to really thrive. So a lot of plant material is going to have to be coming from somewhere outside and accumulating in the cave swamp.

So an underground river might work, except a river strong enough to move a lot of plant material is likely to be moving enough water that you won't get the proper stagnant water feel of a swamp. So I would go with periodic flooding washing a lot of soil and plants into the cave and then leaving it to sit and drain out a lot of water leaving a muddy mix of decaying plants to rot making a pretty nice swamp cave.

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  • $\begingroup$ Thank you for this answer. Two questions: a) Is it really necessary to have decomposing plant matter to have fungi? Couldn't they thrive on other kinds of decomposing biomatter, like guano? b) Also... is it so straightforward that such a strong underground river wouldn't cause stagnant waters? I'm imagining a large shallow basin, like a delta... It could be filled with water overflowing from the margins of that river and be the more stagnant the more we distance ourselves from the river. $\endgroup$ – Pedro Gabriel Aug 11 '17 at 18:11
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    $\begingroup$ bat guano could provide some bio-matter for fungus, but is acidic and if wetted produces ammonia (as in bleach) so large quantities tend to prevent plant or fungus growth. The underground river delta type system could probably work. $\endgroup$ – Josh King Aug 11 '17 at 19:17
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It sounds to me like you do not want a swamp. Swamps are defined by thier ability to support trees, bushes, and other woody plants. A swamp requires a lot of light (think Louisiana, hot tropics, etc.) and wouldn't thrive in a cold, cave-like environment. Marshes are nutrient-rich wetlands that support reeds and grasses, but again, that's more like Florida. It seems to me that what you want is a bog or a fen. Bogs are characterized by their poor soil and high peat content, while fens have less peat and more plant life than a bog. Since peat is made of decaying organic matter, it is plausible that it could drain into a cave. I do not know if peat moss can survive underground with little light. A still-water fen or bog sounds equally gloomy -- although there is no opportunity for mist (no sunlight, no heat, no evaporation, no mist) there's still great opportunities for creepy descriptions. Imagine sloshing through three-foot muck, your path lit only by your make-shift torch. Each sharp splash reverbereates through the cave system, alerting everything that crawls and skitters to your unwelcome presence...

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  • $\begingroup$ Thank you for this answer. Yes, that’s the atmosphere I was trying to convey. Please attribute the imprecisions in terminology to English not being my native language $\endgroup$ – Pedro Gabriel Jan 6 at 8:24
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Both having explored caves, and based on the definition of swamp: "A swamp is a wetland that is forested.1", no, you cannot have a cave swamp, unless the cave has partially collapsed, forming some sort of sinkhole. At which point it's not really a "cave" anymore.

Deep caves tend to have a high moisture content, high enough that spilled water often won't evaporate (90% or higher in most deeper cave systems). And they tend to have soil that makes great mud.

But they don't have plants unless exposed to natural light. And they don't have an abundance of fungi, since they require something to feed on. There are creatures. These Troglofauna have limited resources -- no light means little food.

But cave systems cannot become swamp systems without abundant plant life. That requires sunlight. So beyond the opening to a cave, no.

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  • $\begingroup$ As I stated in the original post, we can bend the classical definition of "swamp" so as to get the desired effect. Namely, "vegetation" may not refer to a forest, but may acount for alternate kinds of flora. Also, fungi does not imply mushrooms, nor do they neccessarily require vegetation (they could thrive on guano, for instance). $\endgroup$ – Pedro Gabriel Aug 11 '17 at 18:01

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