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In many fantasy works, the world typically has giant caverns that house entire civilizations and ecosystems that are (mostly) disconnected from the world above. However, this concept, at least in an earth and non artificial setting, is practically impossible.

With that in mind, what I'm asking is:

How can a system of large caverns form?

Some requirements to base our assumptions on:

  • The cave system is made up of extremely large caverns (similar or even bigger to the Hong Song Dong as to support a thriving ecosystem. I already have a good idea on how the food chain works) which are connected with a series of smaller and more numerous tunnels and crags.

  • Span at least half of a Pangea sized continent if possible.

  • Have multiple, smaller conations to the out side world like little cave openings and rivers.

  • Can last for as long as possible to allow an ecosystem to develop underground. Though parts of the system would fall and crumble overtime of course.

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  • $\begingroup$ See also Journey to the center of the Earth and Ice Age: Dawn of the Dinosaurs $\endgroup$ – Adrian Colomitchi Apr 6 at 4:53
  • $\begingroup$ what about Er Wang Dong ? it say it has its own weather system and has forest too, though i dont know the difference between Er Wang Dong and Hang Son Doong outside the location it originate from, or is it actually the same thing, since from google image it show almost identical image. $\endgroup$ – Li Jun Apr 6 at 7:40
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    $\begingroup$ @LiJun Both cave systems are very close together in Wulong district China and both are part of the same Karst area. There are at least 20 more large cave-systems in the neighborhood. Some are known to be connected by underground rivers. Others are suspected but not researched in full yet. Pictures look very similar as they are the same sort of caves and in some cases pictures are just associated with the wrong cave. $\endgroup$ – Tonny Apr 6 at 11:44
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    $\begingroup$ Mars is supposed to have a fairly large set of lava tubes where scientists suggest the first colonists would probably live. Lower gravity means the caves are less apt to collapse. I like the idea of some sort of organism responsible for expanding and reinforcing the cave structure. You could even have mixes of cave types for different ecological niches. Also consider a honeycomb of small, highly connected caves, possibly left from some biological process. $\endgroup$ – DWKraus Apr 6 at 13:16
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We have to assume that the composition of the soil is solid enough to support giant cave systems across the entire pangea for this to work. The natural formation of such caves seems unlikely. They would be too localized. So how about a semi-intelligent way to build these caves?

Coral reefs are build out of the materials surrounding the life there. With each lifecycle they create a larger structure that supports itself. Something like this could be extended to this world of yours.

Imagine if a similar lifecycle existed in the ground. They create airways to the surface in order to receive oxygen, but grow towards the nutrients and materials they need to survive. These airways are kept open by the life itself as it creates a layer of materials on the edge of the airway they create for themselves. The creatures living on the edge can keep living as they leech nutrients out of the still undisturbed ground, but the things living in the middle aren't that lucky and will die off. These dead parts will then degrade and eventually be consumed by the creatures on the edge which creates a hole in the middle, a cave.

This would encourage them to form an ever increasing cave as the creatures try to increase the size of the cave, but it would form only a small layer and wouldn't support much. So we introduce a symbiosis: Another set of creatures lives in the soil itself. These give nutrients they find to the creatures forming the cave wall, and the creatures forming the cave wall give back something as well. The creatures living in the soil will create a supporting structure around the cave as it forms, allowing it to support more weight.

As long as this "coral" is alive the creatures inside it can gather data from the coral. When we put stress on our bones theres a teeny tiny little electrical difference which our bodies can pick up on and through that guess how much stress the bone undergoes. This information is then used by the body to determine where and how more bone needs to grow to handle these stresses, and where bone can be taken away because it's not used that much and the body wants to be efficient. If this type of land-based Coral would use the same they could keep an eye out for structural collapse. Since a cave-in would kill the creatures living on the cave-in structure and it would potentially cut off a large section of creatures from oxygen they would evolve to prevent this. This causes them to create support structures when they detect a certain amount of stress on their coral structures.

A bit wall-of-texty but I hope it'll give you food for thought.

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There are a few ways in which large caves can form.

The first one is lava tubes

A lava tube is a natural conduit formed by flowing lava which moves beneath the hardened surface of a lava flow. Tubes can drain lava from a volcano during an eruption, or can be extinct, meaning the lava flow has ceased, and the rock has cooled and left a long cave.

A lava tube is a type of lava cave formed when a low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: either by the crusting over of lava channels, or from pāhoehoe flows where the lava is moving under the surface.

Lava usually leaves the point of eruption in channels. These channels tend to stay very hot as their surroundings cool. This means they slowly develop walls around them as the surrounding lava cools and/or as the channel melts its way deeper. These channels can get deep enough to crust over, forming an insulating tube that keeps the lava molten and serves as a conduit for the flowing lava. These types of lava tubes tend to be closer to the lava eruption point.

The other is through karst

Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions.

The development of karst occurs whenever acidic water starts to break down the surface of bedrock near its cracks, or bedding planes. As the bedrock (typically limestone or dolomite) continues to degrade, its cracks tend to get bigger. As time goes on, these fractures will become wider, and eventually a drainage system of some sort may start to form underneath. If this underground drainage system does form, it will speed up the development of karst formations there because more water will be able to flow through the region, giving it more erosive power.

In both cases mind that it's very hard for an ecosystem to form here, since there is poor to no access to some energy source. Typical ecosystems forming in such places rely on wastes being carried through flowing waters, nothing too big or fancy.

Same goes for their extension, they can hardly span over extended areas.

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  • $\begingroup$ For the ecosystem, I’m thinking a mixture of hove ground and a chemotroph based bacteria can be the bases of the food chain. As for range, I’m thinking the caverns connected over a period of time. $\endgroup$ – Seraphim Apr 6 at 6:34

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