I'm designing a world where humans find it easier to travel using arachnoskeletons, exoskeletons that have 8 spider-like legs. I thought that these legs would be the most effective for traversing hilly, rocky or rough terrain.

So, I have a planet (call it Earth II) which has that terrain. It's prime traversing material for my arachnoskeletons. Would there be any specific planetary conditions that would explain why Earth II is so rough? I had a few ideas:

  • The planet is very rocky, and constant storms cause these rocks to bunch up and form rough terrain
  • Extreme amounts of rain in the planet's formative years eroded the surface, giving it a hilly topology
  • Inspired by the Grand Canyon: The planet was initially covered in water which lead to extreme erosion of the underlying rock. Much of the water evaporated, leading to rocky irregular terrain

Other explanations would be appreciated.

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    $\begingroup$ Hello Harith, welcome to Worldbuilding. Please note for future reference that the help center specifically forbids providing your own answers and expecting more. The point is that you already have an answer, so your question isn't serving a purpose. Note that while your existing answers are forbidden, the reasons why you don't like them are excellent conditions for your question so that we're better directed. I robustly and enthusiastically recommend reading through the following two Help Center pages: help center and help center. They cover most of the rules. $\endgroup$
    – JBH
    Oct 30, 2022 at 3:22
  • $\begingroup$ BTW, while I hope you'll receive good answers, it's worth noting that your goal has consequences. A planet with predominantly rocky or broken terrain will have few plains. At a guess, that means the majority of the planet's ecology will be oceanic because there's nowhere for the fundamentals of various biomes to develop (like basic grasses). From a serious make-it-as-realistic-as-possible perspective, a planet that benefits from spiders over wheeled vehicles will be a difficult to colonize. If you're going to handwave that, you might not need an explanation for why it needs spiders. $\endgroup$
    – JBH
    Oct 30, 2022 at 3:30
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    $\begingroup$ Hi @JBH, sorry for that. I agree, it was an oversight on my part. Thanks for linking the guide. I was merely hoping that the community might provide an answer that explained the terrain better / with less suspension of disbelief. I'll keep your advice in mind. $\endgroup$
    – user95290
    Oct 30, 2022 at 20:41
  • $\begingroup$ And my idea for civilisation was that it would be built on -- and in -- the sides of mountains. Your point about grasses is a good one. Maybe hydroponics can come into play. $\endgroup$
    – user95290
    Oct 30, 2022 at 20:43
  • $\begingroup$ Inca empire was weird with wheel usage. You might steal some reasons for why they kept walking despite having roads. $\endgroup$
    – PTwr
    Oct 30, 2022 at 21:08

9 Answers 9


One big problem is that humans have a tendency to make roads. An advanced civilization can just build lots of roads and repair them when they break. You need to make it too expensive to build roads.

The answer is mushrooms

There are massive underground mushrooms which like popping up everywhere. They can grow very quickly and rip through all but very heavily reinforced locations. They tend to have sharp bits in them so they rip up any wheels pretty quickly.

There's lots of these growths everywhere, so any road or smooth terrain that you make is quickly erased. The native species has proven extremely resistant to any sort of fungicide, and so for the foreseeable future wheels are not a useful tool to get around. Spider leg exoskeletons are.

  • $\begingroup$ what about tank tracks or steel rim wheels? $\endgroup$ Oct 31, 2022 at 14:01
  • $\begingroup$ As noted, the fungi tend to rip up any wheels. They have some very hard parts which can scratch even steel. Avoiding them is a lot safer, and arachnid legs can do that. $\endgroup$
    – Nepene Nep
    Oct 31, 2022 at 17:17

Wet Limestone and Big Moon.

The surface of the planet is made of soft rock like Limestone and Sandstone. This is easy to erode and form riverbeds and canyons and glacial valleys.

The planet is plagued by constant storms that moves the water around to erode the soft stone.

There is also a (Edit: Several) large moons with unpredictable orbits and strong tides that pulls the sitting water from one side of the planet to the other, drying up the existing rivers, and creating new ones. This leads to many dry riverbeds and canyons.

The same moon pulls hard matter around the planet on geological timescales. This creates new mountains which leads to new rivers and eventually new dry canyons.

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    $\begingroup$ There is more room for unpredictable or chaotic orbits if there are several large moons on non-planar orbits. $\endgroup$
    – Daron
    Oct 30, 2022 at 16:29
  • $\begingroup$ strong tides and soft stone will make the planet flatter not rougher. you have basically amped up the erosion. $\endgroup$
    – John
    Oct 30, 2022 at 22:29
  • $\begingroup$ @John, that depends on the timescale. If you're coming out of a calm period (snowball Earth? Waterworld?) that built up thick layers of sedimentary rocks, you'll get a period of rugged terrain as the land adjusts to the new conditions. $\endgroup$
    – Mark
    Oct 31, 2022 at 21:10
  • $\begingroup$ @actually that will make it even flatter only coastlines will be rougher, and then only is sea level is rising. $\endgroup$
    – John
    Oct 31, 2022 at 21:27

Geologically active environments tends to be rough. Areas like plains are very old and worn flat from erosion.

Inspired by the Grand Canyon: The planet was initially covered in water which lead to extreme erosion of the underlying rock. Much of the water evaporated, leading to rocky irregular terrain

As far as I know, canyons of this sort tend to exist within flat areas, not hilly or mountainous ones because both the canyon and the surrounding area needed to experience much erosion so the surrounding area is likely to be flatter.

That means that reasoning probably works against you, not for you:

Extreme amounts of rain in the planet's formative years eroded the surface, giving it a hilly topology

Unless your intention is that mountains risen from tectonic activity have been worn down by rain to produce hills, then rain does not produce hills.


You can have


which send out lots of ashes and magma flow in alterations which caves into crazy mountains throughout all the planet.

A lot of lava and ground shaking, weird freaky magma hotspots that cause near-supervolcanos to occur everywhere, where the ground is heaved up by a mile and then erodes, faster than it takes for the water to carve out planes and flatlands.

You can have an atmosphere which causes salty rain and dust in the clouds that add to the mountains just as fast as eroding them.

The geology of the rock and the size of the tectonic plates dictates the amount of mountains. it's like apure geology question.

Find images for "amazing geology rugged" it will give you a lot of ideas for the book.

You will find lots of images and you can search descriptions of how the rocks work which is important for writing a book where rocks are questioned.

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    $\begingroup$ Thank you for the image search prompt. That terrain is a good example of what I am looking for, but I also use landscapes like this as inspiration. $\endgroup$
    – user95290
    Oct 30, 2022 at 20:52
  • $\begingroup$ If the planet has enough super volcanoes to matter its not going to be habitable. $\endgroup$
    – John
    Oct 30, 2022 at 22:27
  • $\begingroup$ Ok STRATOVOLCANOES too. The yellowstone cataclism TV programs are nonsense, earth lived through loads of giant volcsnic mountains... the entire planet could be like hawaii and everest no probs... just shakes a lot like indonesia. $\endgroup$ Oct 31, 2022 at 3:39

you have three easy options

  1. Just use an earth analog, without roads legs usually work better than wheels.

  2. Use a planet with more active plate tectonics, that will keep the terrain active and thus rough. no matter what you still have a lot of flat ground, you can't make a planet with water completely rough.

  3. No grasses, nothing like grass has evolved on the planet so even open plains tend to be covered in too big to drive over plants. you can also combine this with 2 as well.



aa lava



‘A‘ā (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified ‘A‘ā flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface.

Your planet has frequent lava flows. Sharp fields of Aa chunks characterize most places. It is very easy to get wounded by these chunks, even with strong boots.

The other problem is that you might be walking over a place that is thin. Strong boots are also unhelpful if you break through into molten lava. Spreading your weight out over 8 points makes it less likely that you will fall through. Also if one of your spider legs does go through and get lavafied you carry spares.

  • $\begingroup$ ‘A‘ā forms from a particular type of lava. Other types of lava form smooth rocks, or ash beds, or pumice fields, all of which are (relatively) easy to walk on. $\endgroup$
    – Mark
    Oct 31, 2022 at 21:17
  • $\begingroup$ @Mark. Yes. Those other types of lava would not be good answers to this question. $\endgroup$
    – Willk
    Oct 31, 2022 at 21:52

Actions that make a planet more lumpy:

  • tectonics
  • volcanism
  • meteorites
  • extreme tides that distort the core

Actions that make a planet less lumpy:

  • erosion and deposition by water & wind;
  • animal activity (burrowing, over grazing)
  • technological developments

Developments that can thwart lumpiness:

  • horticulture
  • canals
  • roads
  • railways
  • tunnels
  • mining
  • hydrological engineering (dams, either to keep water in or to keep water out or both) effectively using the forces of rivers to create flat land
  • mechanised landscaping (particularly in urban areas)

My gut feeling is that on anything resembling earth, humanity will be able to operate wheeled vehicles within localised areas from fairly soon after arrival, and will eventually flatten the planet. In the meantime if you want to go long distances, flying or boating would likely be easier than spidering.

To have a protracted period without feasible long distance travel the planet would have to be fairly arid with little to no surface liquid (to avoid both erosion and travel by boat), but with sufficient water to sustain humans.

Conclusion: it's an ice world, relying on orbital mirrors to melt localised areas to habitability. Travel between habitable zones would be like cross-country in Antarctica, only with more volcanoes and meteorites.


High winds and sand

Some ideas and problems I see with them:

  • Erosion based: They tend to flatten
  • Volcanic: You have bigger issues than rocks if you have enough volcanic activity everywhere to spew that much out
  • Orbital effects: Irregular orbits or orbits with very large objects will either be unstable over long periods (i.e. you loose your moon) or break up the moon until you have a ring
  • Biological: I really like the idea of fauna (mushrooms or thorns or whatever) to mess with wheels. But I think it's a hard sell that they damage steel.
  • Low gravity: Lower gravity would reduce erosion and make things lumpier in general (think Mars). With lower gravity legs make more sense, because then you can jump across things which I think is neat. But unless your atmosphere is too thin to breathe, flying is just way easier

But a very turbulent atmosphere with a lot of dust & debris is a problem for anything that flies (especially when you introduce sand into jet turbines) and roads because:

  • Roads get buried quickly by sand. Wheels not so good in soft sand. Even tracks struggle in dry soft sand, because as we all know, sand is "coarse and rough and irritating — and it gets everywhere."
  • If it's always dusty, then you can't really drive very fast in any case, so the penalty of legs vs wheels is reduced
  • 8 legs with blunt ends distribute your load nicely, and are tolerant to hidden rocks under the sand


wombat burrow


Your planet has creatures that dig burrows. These burrows then collapse. There are several types of these creatures, from prairie dog size to wombat / aardvark size to much larger creatures like bulettes.

The result of all of their activity: the ground is extremely irregular with holes, collapsed tunnels, and more solid looking places on the verge of collapse.

The spidersuits can get a person past the smaller types of collapses. Large but shallow burrows might be detected by a sensor scanning the ground ahead - falling into one of these would mean a fall of some meters and that is if the burrow is perpendicular to the surface. If the burrow is inhabited the fall might be just the start of your troubles.



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