So I have a cube world with the standard 6 sides, with a top and bottom that do not change. We can ignore any rotation of the other side-sides for the sake of this question. I am trying to come up with a plausible water cycle for this cube world, but I'm having trouble due to a quirk I've written into the world: the gravity a material experiences is completely different depending on its state of matter.

  • Solids experience gravity "perpendicular" to the face of the cube they're on, uniform from the center to the edge of the cube. Over the edge there is a transition zone where the gravity curves sharply around the corner, but if there is ground directly beneath you the gravity will be normal.

  • Gasses experience "normal" spherical gravity towards the center of the cube, meaning there is more atmosphere above the center of each side than the edges, with the corners being the most inhospitable for any creature that needs to breathe.

  • Liquids are by far the strangest though, experiencing "absolute" gravity that falls in a single direction regardless of which face of the cube one is on. That direction being the "down" as if the cube was resting on a plane. So at the top of the cube water would fall towards the surface, on the sides it would fall parallel to the surface, and the bottom only sees the spray of water as it falls up and away into space.

Right now I have the top face of the cube as an ocean, with the water that spills over the edges of the cube flowing down the sides as rivers, which then continue off the bottom edge of the cube into the infinite void of space. Assuming there is an infinite source of water filling the ocean at the top of the cube this isn't a problem, but I started wondering, could there be some mechanism to reclaim this lost water to give the world a closed water cycle?

So far I have considered that at some point the water would disperse into vapor and be pulled back towards the cube due to the gravity of gasses, but I'm not sure how this would play out given rain can only ever fall back towards the bottom of the cube. I've also considered some sort of condensation-based pump system that pulls water from the air at the bottom of the cube and flows it back to the top like a planetary water fountain, but this doesn't feel scalable to an entire planet's water supply.

Behold, Art!

  • $\begingroup$ I'm not clear what the question is, it seems more like a discussion prompt than a focused worldbuidling problem for us to solve. Can you clarify? $\endgroup$ Commented May 18 at 10:12
  • $\begingroup$ Humans being made of water one wonders what would happen if they actually did try to cross faces ... $\endgroup$
    – BMF
    Commented May 18 at 11:39
  • $\begingroup$ I am looking for a plausible way for the planet to have a water cycle that involves recapturing the water that is currently being lost falling off the bottom of the planet, and ideally bringing that water back to the top to create a closed water cycle. $\endgroup$
    – IVIX
    Commented May 18 at 15:23
  • $\begingroup$ There are many gray zones of materials being partially affected by multiple gravities, such as humans feeling the orientation of the liquid gravity (gives them a great sense of direction, but they do not fare well on the bottom face), despite being solid simply from the liquids they drink. Even weirder is blood, which is treated as a solid when inside the body and even for a short time outside, until it cools and begins flowing as a liquid. Lots of little irregularities in this world, but that's outside of this question's scope. $\endgroup$
    – IVIX
    Commented May 18 at 15:32
  • $\begingroup$ The edge case that may be very relevant, which I haven't nailed down mostly because I'm trying to solve the whole water loss problem, is water. It shifts between states so readily that every time I consider the implications of ice my brain breaks a little more. $\endgroup$
    – IVIX
    Commented May 18 at 15:38

1 Answer 1


Short answer Assuming that the only forces moving the water is gravity and the water interacting with its self (surface tension, air pressure...), then there are 2 ways it could happen.

  1. water becomes gas then flows back, by atmospheric pressure.
  2. water becomes snow and pushes itself back

Longer answer (may contain science)

The behaviour of fluids (like air and water, and on large enough scales solids too), is very much shaped by the gravitational field it is experiencing. In general its overall shape can be determined from an concept called equapotentials, equapotentials are 3D surface where the potential energy from a force (in this case gravity) is the same. For Earth and its radial gravitational field the equapotentials form spheres, like layers of an onion, hence the air, water and crust on earth form approximate spheres, there is some stretch around the equator because of the earths spin.

The equipotentials for your 3 fields would be quite different, For air it would form the familiar spheres as it slowly diffuse into space. For liquids it would form lines stretching down to infinity For solids it would for cubical shells like a "minecraft/blocky" onion

Since as you say, you don't want the water to fall of into space, so it needs to change state (which could also be describes as a "phase shift") so it can be pulled back towards the planet, this produces 3 cases.

Case 1, the falling water could vaporise/evaporate into water vapour, that flows back to the upper part of the cube to condense into a cloud, rain and complete the cycle.

Case 2, the falling water could freeze into snow, which settles into drifts, that grow into an ice sheet that grows back to the top of the cube where it melts, completing the cycle.

case 3,a mix of cases 1 and 2, the ice sheet forms on the bottom like case 2, but vaporises as it crosses onto the sides forming water vapour and contimuing like case 1.

So exactly what could happen would depend on what the climate of you world, case 1 would need the bottom to be very warm/hot, with the top being cool, case 2 needs most of the cube to be cool/cold, case 3 would need the bottom to be cold, the sides to be very warm and the top to be cool.

Hopefully that helps

  • $\begingroup$ This is exactly what I was looking for! The piece I was missing was your blurb about climate. Case 1 is the most promising, since I was already thinking of the bottom as a fiery hellscape due to magma (liquid-solid) pooling at the bottom of the cube. So water falls off the bottom of each side into space, then evaporates/vaporizes into gas, which is pulled back towards the surface. The wet air flows from the hot bottom to the cool top and falls as rain. Rinse and repeat. This would probably allow for the odd sideways storm on the sides. $\endgroup$
    – IVIX
    Commented May 20 at 12:36

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