I'm doing a Pratchett style fantasy book, and the setting - the Downspout - is built in a downspout of an astronomically sized gutter drain pipe.

We're assuming an impossibly large structure housing impossibly large creatures - perhaps a giant Ringworld housing a species who built it known as Titans who are astronomically giant. I'm not interested in feedback about that, as Titans won't play much role in the book aside from being deities to some species at our scale. It's just a Pratchett style high concept.

On the side of the old pipe of a Titan building, a fungus grew into the side of the steel pipe using rust and carbon from steel and drain water with dirt, and eventually the fruiting body grew to the size comparable to Earth. So 10^7 square meter surface area order of magnitude, with the pipe diameter being around 30^8 m range - comparable to distance from Earth to the Moon. Building itself is about 60 pipe diameters by 60 diameters, with about 40 pipe diameters tall, I have no idea what materials it could be from honestly. (So if my math is correct, building is 180^9 m long and deep, and 120^9m tall). The building is mostly empty and I am not sure how thick the walls would need to be. While the drain pipe is carbon steel, the walls of the actual building could be made from exotic materials if needed.

Some species are on the same scale as Titans, while some exist at similar scale to humans. The fruiting body of the macromushrooms is shaped not dissimilar to chantarelle mushrooms, with dips in the middle - that would naturally create seas in the middle of the mushroom, and 'mountains' at the edges.

This resulted in fungus being habitable, and over aeons life has evolved there, at our scale - plant seeds washed down from above, and eventually humans, elves orcs and kobolds. That's the setting of the novel.

However, I have to admit I have zero idea how a water cycle would work in such a setting. My best guess so far is that sea forming in the dip in the mushroom could evaporate due to sun from above, and condensate back down on the bottom of the cap above it - and that would be rain, feeding the rivers from the mountains at edge of the mushroom back down to the sea. That has worldbuilding implications, since e.g. life could only exist on the mushrooms positioned vertically in the middle of the cluster, since very top mushroom won't have a water cycle.

I also realize that since it is a drain pipe, naturally water will come streaming from above after every major rain in ginormous quantities, which is something the civilizations will have to adapt to (another reason that living in the middle of the mushroom cluster sounds optimal, since top mushrooms will break the stream, resulting in less damage and more usable water).

I'm also not sure how gravity would work at this scale - Ringworld has its own gravity 'downwards', but so does the building and the mushroom. I'm not sure how gravity would behave with objects at this scale - would the building itself have a gravitational effect at this scale?

I have no idea if that's correct. I'm literally a high school drop out.

So any help regarding trying to picture how a water cycle on that scale would work, and if gravity of the building would affect living in the Downspout.

  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    May 22, 2023 at 19:02
  • $\begingroup$ Since you mention PTerry, probably the best thing to do is observe how he resolved this problem, by reading Strata. Question: have you read that? $\endgroup$
    – elemtilas
    May 23, 2023 at 1:24
  • $\begingroup$ For what it;s worth 60 times 30^8 is 2 * 30^9 not 180^9. $\endgroup$
    – Daron
    May 26, 2023 at 12:29

3 Answers 3


Treat as a pure fantasy

In the real world, any body over a certain size will form a sphere - which is why you see some weirdly shaped asteroids, but all planets and stars are spherical. (Yes, there will be equatorial bulges from rotation, but overall the body will be spherical.) The more massive the body is, the closer to a perfect sphere it will be, because gravity will be stronger.

While there are some proposed megastructures with theoretical materials such as ringworlds and Dyson spheres, these rely on their component parts each being effectively in orbit around their primary (star) to prevent the structure collapsing. It is simply impossible in a hard physics setting to have a megastructure warehouse with a drainpipe and a planetary-size fungal mass in it, all presumably on some ridiculously massive planet. (If you did, then "down" would be towards the centre of the big planet the building is sitting on, the same way that on Earth "down" is towards the centre of the planet, no matter how big the building next to you is.)

However, none of this prevented the late, great Sir Terry from using the Discworld as the setting for many of his novels, as you noted. He simply ignored and/or handwaved physics for the purpose of his stories and it worked fine, occasionally throwing in comments about how things differed from other worlds made by gods that were more picky about consistent physical laws. (I remember that the seas flowed off the edge of the Discworld, but cannot even remember if he had a mechanism for that water being recycled into rainfall and I do not care.) If you tell a good enough story then no one will care about the fantastic nature of the setting - astronomers don't go around gnashing their teeth about Discworld contributing to scientific delinquency. (Not the ones I have met, anyway.) Let moon-sized leaves go fluttering past the fungal world every so often in mega-world autumn, with one striking the fungal world every so often and creating earthquakes with spores dispersing, or whatever you like.

In summary: You cannot make a "world" like this scientifically plausible, so don't try.

  • $\begingroup$ Yeah, I guess. I knew this will come to this. I wanted to have some amount of plausibility, but I guess at the end of the day I have to reconcile that I'm writing a fantasy novel, not hard sci-fi, and this concept is too surreal. Thank you! It also made me realize that while Sir Terry used science logic in his worldbuilding, it was never exactly actual science. So I just have to make it sound believable, instead of being realistic. $\endgroup$
    – shinobody
    Jun 4, 2023 at 0:46

hmmm, I would suggest scaling the world down. The mushrooms being the size of a very large city (in terms of cap width), the downspout being earth-sized in diameter.

I don't know how to justify the titans, but I assume they are beyond needing justification really. Don't let hard science get in the way of a good story, it's nice to base things on some sorta grounding but that isn't appropriate for this setting. I mean, Terry Pratchett has that discworld which is towed around space by a turtle. I haven't read that book, but I know- despite it lacking in hard science- it is seen as a highly regarded science fiction book. However, how I'd do things, I'd say that physics within the pipe are somehow scaled down. Within the downspout, physics act as if the pipe's the width of earth, but outside it's the size of a downspout, and the titans are just the humans we know.This requires a different handwave, but I'd argue this would be more interesting.

The downspout doesn't collapse into a sphere because only the inner part of it is at the smallscale physics, most of it is bound to the largescale physics and the inner part is of course held to the outer part via the normal forces that hold a material together (weak force or something?), and the smallscale-largescale boundary downscales those massive planet-forming forces into a light load, less than a newton, that the galvinised steel pipe doesn't get bothered by.

Though Flushed Away (kid's movie with the rat people in the sewer) isn't THAT similar to this, and I assume you're going for an older audience, there is clearly a desire for little-people-live-in-our-world sorta stories, because they're fun! It's fun to think about an entire city, country, or world built right under our noses! (see also: Horton Hears a Who, for an entire microscopic city that is oblivious to the outside world. Once again, young target audience, but the basic concept of a world oblivious to the larger one would still carry to the adult market)

Now, water cycles. The heat on the side of the pipe, from the hot sun, could cause hot, humid air to rise from the pool at the base, under the mushrooms.

This humidity rises up the pipe, and will later trickle down- most drops running along the sides of the pipe, but a curved section of pipe causes water drops to lose their surface tension- this pipe curve is rough due to corrosion and thus gives the drops a batter chance to fall.

I was going to say that the drops would be massive and would cause injury and property damage, but because this drain pipe is earth sized I am sure the droplets are the same size as our raindrops, so this is not a concern

I hope my suggestions don't ruin your story. Your story may depend on something that is completely broken by this suggestion. I hope that I've given at least some good advice anyhow. I hope my water cycle idea is at least solid enough for you.

  • $\begingroup$ Wow, that's a good point. If the pipe is made from steel, why didn't it collapse? If I turn pipe to be made from exotic materials (which is reasonable, everything else seems to be made from it) then I would have to come up with a new source of metals for the people of the mushroom. I don't think I would do it exactly - I'm definitely not turning titans into humans - but I could make it so that titans use magic or something else to change laws of physics on the ringworld, and downspout is a localized singularity with different laws of physics. Thank you so much! $\endgroup$
    – shinobody
    Jun 4, 2023 at 0:42
  • $\begingroup$ Happy to see that I helped! $\endgroup$
    – Katy
    Jun 5, 2023 at 7:32


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Much like trees, mushrooms take up water with their roots and excrete water vapour. For mushrooms the roots are called mycelium. Your big big chantarelle has big big mycellium that extend around the inside of the pipe. The pipe is forever wet from Titan poo poo and pee pee.

The mushroom is positioned such that the poopoo and peepee never hits it diectly as this would be a world-ending phenomenon for the tiny peoples of St. Mushroom.

The tiny people do sometimes observe the poopoo and peepee as their equivalent of astrological phenomena. They use it to predict the future.

Instead of smashing the mushroom, liquid from the poopoo and peepee wicks through the tiny cracks in the inner surface of the pipe towards the mycellium. The mushroom loves that. It eats up the poopoo and peepee and creates clean water for the tiny peoples to drink and swim.

The water cycle is not a circle. It is a straight line. Water falls from above, is sucked up and purified by the mushroom, is used by the little people. Eventually it evaporates and leaves. Then another Titan comes back for number 2.

  • $\begingroup$ That is somewhat helpful but in case it isn't clear - I meant like, roof gutters and drains. Not sewage pipe. So main water source is overworld rain, not titan waste. Still, this did help me so I will still upvote it. Thank you! $\endgroup$
    – shinobody
    Jun 4, 2023 at 0:39

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