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I have a worldbuilding question about maps,

when you are making an underground map (of a scale like continents worth) what are some ways to make it less homogenous (aside from aquifers and water and such, I am planning on having those, but I dont know what to add besides that)

Sorry for answering this so late, I am planning on doing a better post about this, but I hope this will be helpful!

Lately I've been also wondering about "underworlds" and such underground cavern systems, how would they work? Are underworlds like in Journey to the Centre of the Earth possible? First, I think it's a good idea to understand how they work in real life.

Big cavern systems in real life are basically created in two ways: "solutional" caves, that is, caves created by rock dissolved by water, often limestone or other carbonaceous 'soluble' rock (which, interestingly enough, often come from sediment made from the shells and calcium skeletons of ancient marine life), and "primary" caves, often called "lava tubes"; these are formed when lava flows through "harder" rock, leaving empty "tube" structures on it. Mars has many of these and they are being considered for habitation in future missions. There are other kinds of caves made by sea erosion or glaciers, but when one thinks big cave systems, these are the ones.

The shape of a cave usually depends then on the "hardness" of the rock, that is, how much of it is dissolved by water. The classic shape is a branched one, like rivers or tree branches, but depending on the type of rock, you can get some that are more like sponges or shattered in structure. However, cave systems depend very much on the rock available, big cave systems are found where big limestone and such rock are found (China and parts of the US and Europe have the biggest, while they aren't really found in South America for example).

There does seem to be, according to Wikipedia, a limit of 3000 meters of depth to caves before the pressure of the rocks above makes their formation impossible. Also, again, karst deposits are found usually in the upper layers. I do wonder, for example, in a fantasy world with different rocks and minerals, if you could have some kinds of rocks that could dissolve more in water, while still being hard enough to sustain the pressure from the rocks above.

One question I had is that if you could have "vault-like" caves, with underground lakes or oceans, like in classic fantasy. The largest such chamber found is the Miao room in the Gebihe cave system in China, which is a respectable 852m in lenght and 191 in width. And I'm very sure there are many more not found. However, these are very far from the huge underground vaults of fantasy, like the sea from Journey to the Centre of the Earth:

For the reasons stated above, I think these structures, if possible at all with the mechanics of the rocks, would be very vulnerable to erosion and collapse, not to mention earthquakes and tectonics. Realistically, you couldn't have a sea with ancient plesiosaurs and such surviving millions of years.

In fantasy, of course, you can think of believable worldbuilding around this. You could very well have gods or other beings to create these caves, and it has a very poetic ring to it, the "pillars of the world". You could have volcanic intrusions or dikes that are then dissolved by natural or magical means, resulting in these vault-like structures. So if you want to think about geography, instead of continents or big rooms, I would rather imagine archipelagos of caves, connected by tunnels, sometimes in different layers.

Another thing to consider is temperature. Caves are thought of cool places, in fact, they're just very insulated and dark, so their temperature is stable. However, the deeper you go, the more temperature increases. Temperature increases roughly 25-30°C for every kilometer, more if the area is geologically active. Deep mines, for example, need air-conditioning and even then they are hot, humid, horrible places to work. I think the habitable underground zone would be no more than one or two km deep.

Now, the most interesting part is of course, what does life look in caves. There's plenty of life adapted to caves, but what really strikes me is that the supply of nutrients and energy comes from the outside ecosystem rather than the cave itself. There's plenty of guano in bat-filled caves for example, but the bats of course hunt on the night outside. Caves aren't very productive environments on themselves, they don't have many options for producers, that is, the first link in the food chain that. Of course they're dark, and while there's some very extreme algae and bacteria that can photosynthesize and very interesting chemosynthetic ecosystems, like those found in oceanic undersea vents. Perhaps the most extreme case is that of Movile Cave in Romania. It's a fully contained ecosystem based in chemosynthetic bacteria, whose biofilm supports fungi and others, and they support a complete system of endemic invertebrates from crustaceans to mollusks. I really cannot do the uniqueness of this justice, so I'll just link to the Wikipedia article.

(it's likely that there are many caves like this not yet discovered!)

However, these ecosystems are very, very fragile, and very, very slow paced. Again, there's lot of organisms that adapt to caves, but the tendency is almost universally, besides losing pigmentation and vision, to slow down activity and metabolism as much as possible. The most emblematic example to me is the olm/ulm, a cave salamander that reaches a respectable half a meter, the apex predator so to say, can spend months or even years without moving, only for feeding and mating. Same for example with the growth and reproduction of many other animals in caves, or for that matter, fungi or microbes. Everything is done to reduce metabolism to a minimum.

For a fantasy setting, assuming creatures who live and build cultures in this underworld, this has implications. Sentient creatures need a lot of energy, a lot of calories. What do they eat, what does their lifestyle looks like? I've seen some worldbuilding (like in Age of Wonders IV) assuming fungus forests, but what makes those fungus grow into forests instead of just biofilm? Unless there's some magical means introducing light and nutrients into the environment, some kind of crystal or bioluminiscent fungus or a base producer like some really good biofilm, one has to wonder what the base of the food chain looks like.

Instead of seeing this as a limitation, I think this is a great chance for writers to do something original and explore this! Just think about how different a cave ecosystem could be. And what's more interesting to me is to imagine the flow of nutrients and energy with the outside world... bats, fishes, salamanders, cave dwelling animals, and of course, peoples...

I'm already writing way too much here, but I would like to finish with an awesome video video which expands on what I've said and also has a very interesting conclusion, that salamanders are uniquely predisposed to take the top spot in the food chain of cave ecosystems!

Also, this IS NOT a demand at all, but if anyone reading loved this post and would like to see more, you can always check out my #biotipo worldbuilding tag, and give a tip to my ko-fi! I really like doing these kind of posts even if I take a while to get to them, and I hope you find them interesting!

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Don’t have combat.

have combat rules but if a player starts combat they must retire their character immediately after combat ends.

Don’t bother trying to stop them. It’s impossible to fool proof any game so you may as well write it for the players that “get it” not those that won’t.

Explicitly have players create NPCs with relationships to their PCs with the friendliness of that relationship determined by a stat. Combat of any kind (or most at least) results in penalties on all relationship stats. Killing changes a person and their loved ones would notice.

combat rules are: flip coin. Success, enemy dies. Fail, PC dies.

Ways to discourage players from murder hoboing in my ttrpg?

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Oh I know where these are! This is the madrasa Ben Yousseff in Marrakech, Morocco! There are several ladders like this one in the rooms for students.

Well, this is awkward.

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Art for my ttrpg scenario The Old House in the Corn That Only Children Can See

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