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I had a unique idea for an alien planet. The surface would be snow-capped mountains and glacial fields, it's cold and essentially devoid of most life. But just underneath the towering mountains would be this whole other world. A vast interconnected web of caves, tunnels, and hollows that essentially act as a planet-wide terrarium. There are even openings allowing sunlight to stream in allowing photosynthetic lichens to grow, forming the base of this world's food chain with creatures feeding on the lichens and creatures feeding on the creatures, feeding on the lichens. these openings also play a part in this planet's water cycle with water vapor evaporating through some of the openings like vents, only to freeze and fall back down as snow onto the mountains and glaciers, for the melted runoff to flow back into some of the openings starting the whole cycle over again.

But while this is an exciting idea for a setting, I am struggling to answer the question that interstellar explorers will undoubtedly ask when they find this place. How the heck did all this happen? So once again I am calling on the wisdom of the crowd to answer this question.

What scientifically plausible explanation, explains how this vast nexus of hollows, caves, and a thriving underworld on this alien planet come to be?

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    $\begingroup$ Your biggest challenge here is that you want openings big enough for sunlight to stream in and power photosynthesis, but not big enough to let the glacial ice and cold air in. I think you have to tweak that a bit. Maybe windows or domes of crystal clear ice are laid down by some unusual process, which act like igloos over the terrain beneath? $\endgroup$ May 26 at 12:04
  • $\begingroup$ That might work. Certainly would make the planet mor unique. $\endgroup$ May 26 at 17:11
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    $\begingroup$ Where is the energy coming from? No energy-> no life. A few acres here and there of sunlight would not be sufficient for a 'vast nexus' of ' thriving underworld' $\endgroup$ May 26 at 21:44
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    $\begingroup$ Can you first say how any hollows, caves or any other underworld features might be a problem? $\endgroup$ May 27 at 0:11
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    $\begingroup$ I'm afraid this is yet another planet that's the same everywhere. A part of a planet might be as you describe. Probably a layer of hard rock which has eroded through in parts, exposing a thick stratum of limestone underneath. Acidity in rainwater has created an extensive system of caves. You can find places like this on Earth. Scale it up a bit if you want, but to global scale strains credibility well past breaking point. $\endgroup$
    – nigel222
    May 27 at 8:54

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The caverns grew.

coral caves

https://www.globotreks.com/destinations/mexico/diving-in-cozumel-top-dive-spots/

Coral reefs form caves as they grow. On your world this was more extreme. Back when this world was warm and full of life, coralline land forests grew and ramified layer upon layer. The old was buried under the new over the ages as coral reefs are today.

As the world cooled life adapted. There had always been life down in the caves - low light lampenflora at higher levels, chemotrophs farther down - and these life forms persisted as the upper world froze.

The land coral trees are not dead but it is cold up top now and so they no longer grow. They bide their time.

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  • $\begingroup$ I think the coral-like super organism is a good idea, but I am curious about weather iron and other metals could still be mined from the cavern walls. $\endgroup$ May 26 at 22:52
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    $\begingroup$ @JacobBadger - they can have metals if you make that be the case - maybe these organisms when alive concentrated minerals in nuggets like pearls? $\endgroup$
    – Willk
    May 26 at 23:04
  • $\begingroup$ I like it, digging out pure nuggets of the stuff would be like a gift from the stone god. I had a crazy thought of adding a biome that would essentially be a forest of pine-tree like lichens that photosynthesize in the lowlands below the timberline and share their glucose and sugar with the mountain-coral in exchange for nutrients from the underground. Earth corals have a similar arrangement with algae, so maybe this could work. $\endgroup$ May 26 at 23:15
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    $\begingroup$ @JacobBadger - also trees and mycorrhizal fungi. I dig it. $\endgroup$
    – Willk
    May 27 at 1:13
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    $\begingroup$ @JacobBadger FYI, the principle source of iron ore in the pre-modern era was bog Iron which is an iron ore that has been consolidated by organic means. $\endgroup$
    – Nosajimiki
    May 27 at 20:56
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Limestone

The biggest cave networks om Earth by far are all limestone caves, created by karsts (basically, self perpetuating sinkholes).

Wikipedia offers this: "Preferable conditions for karst cave formation are adequate precipitation, enough plants and animals to produce ample carbon dioxide, and a landscape of gentle hills"

The following link has some starter diagrams about karsts: https://www.nps.gov/subjects/caves/karst-landscapes.htm

If your planet has a ludicrous amount of limestone and/or dolomite, you have a good chance of getting great cave systems.

If it's sci-fi, maybe you could have some sort of life-critical element or resource (iron? water?) that sustains creatures that dig out or expand tiny tunnels.

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The planet could have a system of lava tubes. They are known to occur on Earth, the Moon and Mars. Some of the larges lava tubes are in Undara Volcanic Park in Queensland, Australia. Some tubes break through to the surface to create an entrance and allow flora and fauna to reside in them. They have even been considered as initial residential locations for habitations on both the Moon and Mars.

Depending on how close the tubes are to one another, it may be possible that some interconnect naturally, but in other situations, artificial tunnels can be established between tubes. There sizes can vary from small to very large: several meters in diameter and hundreds of kilometers long.

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In addition to the other answers regarding cave formation. Ice-Ages:

enter image description here

CCASA 3.0, via Wikipedia 2022

Life developed in fits and starts on our Earth. For the longest time there was water and single celled life, then the ice came. From this sprang forth photosynthesis.

After the Huronian glaciation, life spread and multiplied (literally, that's when sexual reproduction happened for the first time as far as we know). Then multicellular life, fungi, plants and the first animals.

Another explosion of life occurred following a subsequent ice-age, then another resulting in tetrapods, then another - eventually life became complex enough to design websites like this one. Life had spread and colonised the lands and the seas, including cave-systems and nearby niches.

Then the ice returned, big-time. No large photosynthesizing plants or animals comprising an ecosystem could survive on land in the open - many migrated, seeking shelter in the caves to become fertiliser for the life already there.

TLDR.

Life developed similarly to the Earth's, then retreated to the caves during a long ice-age. There'd still be ice-algae on the frozen surface and life beneath the almost-solid ocean sustained by deep-sea vents and chemosynthesis. The advanced lifeforms (who expanded the system) died out leaving the basics of life, and some traces..... perhaps. Maybe some made-it off planet too.

Reason:

The last part - a long glaciation period - perhaps a celestial object entered the solar-system and perturbed the orbits, just enough to perpetuate the lower temperatures.

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Gas Bubbles

When the planet was first forming, there was a greater proportion of gaseous materials relative to solid materials than Earth. When the Earth formed, the proportion of gaseous materials to solid materials was low, and hence you get a solid planet. When Jupiter or Saturn formed, there was much more gaseous materials than solid materials, so you get a gas giant planet. This your made up planet has a proportion of gaseous materials to solid materials in between that of the Earth and that of Jupiter.

When the planet was forming, the gaseous materials intermixed with the solid materials. And when the planet was hardening, the gaseous materials got trapped in between the solid materials, creating gas pockets underground. The solid materials weren't able to fully coalesce into mountains of solid granite because of the gas bubbles. The gas pressured against the stone, and didn't allow it to fill up these empty spaces. So instead the continents became full of these pockets of empty space in the middle of stone.

Because of these gas bubbles, your planet resembles a piece of swiss cheese, a rocky surface with lots of gas bubbles. There are lots of empty spherical hollow spaces in the planet, ranging in size from a living room to several miles in width. Those gas bubbles that were on the surface, or close to the surface, turned into caves, and in some places where two gas bubbles were closer together, the rock barriers collapsed under their own weights, connecting these empty spaces. Although there still remain a lot of unbroken gas bubbles buried deep into the crust of the planet, surrounded by rock on all sides.

Over time, lava, water, ice, plants, and animals made tunnels connecting these empty spaces, perhaps if you have relatively large burrowing creatures. Then later in history these empty spherical caves were settled by people, who reused some of the naturally formed tunnels, and also created a proper tunnel system between the various empty pockets, like a subway or mine carts.

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  • $\begingroup$ I think this one might be the winner. I’ll give it a few more days, you know be fair to everyone but I think this might be the definitive answer. $\endgroup$ May 26 at 17:43
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    $\begingroup$ Unless the planet is less than a couple of hundred km in diameter, it simply won't work as the forces are such that the rock acts as a fluid under gravity and the gas will be let out - unless you make it a very lightweight planet made of something non-naturally strong. It would work for a science-fantasy planet just fine though. @JacobBadger $\endgroup$ May 26 at 22:11
  • $\begingroup$ You could have a few vertical km of tunnelled earth easily enough though. Even a few km will mean the tunnel area is greater than the surface area. FWIW, lava that is almost pure silica is very viscous and hardens to be very strong. Wouldn't have much in the way of nutrients, but maybe that stuff got spat out on top and dribbled in via water, etc. $\endgroup$ May 28 at 20:33
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Geothermal vents first opened passages in the rocks, then when the heat was not sufficient to boil the water and form geysers it just left a flow of warm air, sufficient to contrast the freezing cld from the outside.

Something similar happens in some thermal springs around the world, where while the surrounding is covered on snow enjoy mild conditions thanks to the pond of warm water. Just scale it up.

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(10's of) thousands of years ago the planet was inhabited by an advanced (pre-warp) species; a cataclysmic event made the planet's surface uninhabitable, forcing the survivors underground; initially habitats were built within pre-existing caves and domes but with the development of 'underground' technologies (drilling, boring, structural designs, geothermal power, etc) an extensive network of caves, tunnels, and hollows was created; the development of crystalline building materials allowed for the construction of near-surface biodomes in which various species of surface life (plants, insects, small animals) could be maintained in the hopes of one day returning to the surface and reseeding nature; [you could add in here some technologies that were developed to route geothermal energy to the surface to facilitate the melting of snow and the subsequent collection of the water/runoff]

Many generations passed and the surface conditions remained uninhabitable; with time the 'memory of the ancestors' was lost and with this loss came the degradation and eventual failure of the technologies on which the majority of the species depended; fast forward to 'today' and what's left are the remnants of the great underground structures, with the few remaining descendents of 'the ancestors' eeking out an existence in the few remaining and still functioning crystalline-topped biodomes.

Yeah, basically a rehash of the civilization-dies-and-starts-over-from-scratch that's been used in an assortment of sci-fi series and movies (eg, the 100, bsg, stargate, see, etc); plenty of room to expand the backstory as well as flesh out the current inhabitants (pre-tech? barely getting by with spit-n-bailing-wire fixes to remaining tech? comfortably living in bunkers and raised on dangers of leaving the biodomes?); and of course there are plenty of options on what the interstellar explorers find when they arrive and how they proceed from there, eg, discovery of some super tech developed by the 'ancestors' (ala stargate), or perhaps a biological marvel that's allowed the inhabitants to survive this long, or, or, or ... or maybe the original inhabitants are long gone and the only thing still alive are the lichens, and creatures feeding of the lichens, that survive in the remaining biodomes, and the newcomers get to play archeologists ...

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