Acording to my understanding of the word "swamp", we must have at least 3 components put together:
- Water in abundance
- 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?
High water content is not a problem, since there are many instances of water in caves, sometimes even entire lakes and underground rivers
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.
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?