Having escaped Isengard and left Rivendell, we are off to destroy the Ring. Our first stop--Khazad-dum, known contemporarily as "Moria". Being a Dwarvish kingdom, the problems I see with Moria are identical to those with Erebor in The Hobbit. Hollowing out a small piece of a mountain to build a mine is no big deal, but the dwarves have hollowed out the entire mountain to build a kingdom. In real life, that would be turning Everest, the second-tallest mountain in the world (if only Mauna Kea weren't an iceberg), as hollow as a discarded snail shell.

In real life, such things would have the following problems:

  1. Hollowing out the entire mountain would release a great deal of pressure, and if the landslides at Yosemite are any indication, that would be one-sidedly disastrous.
  2. Caves are viable habitats, more viable than lots of people would imagine, but they're still not verdant enough for the use of agriculture. One cave in Texas, for example, has a population of only 100 blind salamanders.
  3. Perhaps the most obvious problem is light. You could argue that we could build windows everywhere in the mountain, but we have to bring #1 to mind.
  4. This problem would be exclusive to those who have a fear of heights (like me.) Living in a mountain-sized subterranean city would be daily vertigo because of these vast gaps.

In order to make subterranean structures on the scale of both Moria and Erebor a reality, how would those four issues be addressed?

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    $\begingroup$ Um. Where does it say the "entire mountain" is hollowed out like a snail's shell? Sure, a Dwarf mine is nothing like the dark dusty tin mines of Cornwall, and sure Peter Jackson made the places visually overextravagant, but I don't think Tolkien ever any of those places as entirely hollowed out. $\endgroup$
    – elemtilas
    Dec 17, 2017 at 18:39
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    $\begingroup$ I had always thought that a dwarf mine started as an outpost, with several dwarven farmers providing food for few miners and prospectors from the very outskirts of the mountain, and if the mine ended being really prosperous they quickly reach a point where they have to buy the food from an external source - Esgaroth in Erebor, Eregion and the kingdoms between Lothlórien and Mirkwood in Moria. $\endgroup$
    – Rekesoft
    Dec 18, 2017 at 11:29
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    $\begingroup$ @JohnWDailey Not quite: do not forget that the dwarves dug very deep down, below the ground level. They are not just digging straight in and up. So you have made a false assumption with envisioning a hollow mountain. Somewhat ironic that you yourself mentioned an iceberg, so think of it that way; the majority of the iceberg is beneath the surface. $\endgroup$
    – Loduwijk
    Dec 18, 2017 at 19:27
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    $\begingroup$ @Rekesoft Per the Sillmarillion, Aule made the dwarves after he had already made a pretty sizable mine, so devine-like intervention was involved at first. Second, the Dwarves spent thousands of years making mines before the events in LotR. Combined with the other noted misconceptions, there doesn't seem to be a problem for Tolkien to solve. $\endgroup$
    – user458
    May 16, 2019 at 20:26
  • $\begingroup$ Orthanc was made by the Numenorean with a substance so hard that even the ents in their furry can barely scratch it. I bet dwarves have stone working techniques equal or superior to that, allowing impressive pieces of architectural work. I'm afraid a reality check doesn't apply to Middle Earth. But this doesn't invalidate the question $\endgroup$
    – JFL
    May 17, 2019 at 20:19

5 Answers 5


JohnWDailey - Again I say there is a big difference between Dwarf and Goblin/Orc underground architecture in the books and in the Peter Jackson movies.

Suppose that Moria had a population of 1,000,000 Dwarves at its height.

Using farming techniques 2,350 persons could be fed with the yield in sweet potatoes from one square kilometer of land. 425.53 square kilometers of land would be needed to feed 1,000,000 Dwarves - equal to an area 20.62 by 20.62 kilometers. Suppose that Moria had a series of chambers 5 kilometers by 5 kilometers square, stacked one on top of another with 100 meters between each level. 20 levels would reach 2 kilometers high and would contain 500 square kilometers of farmland, more than enough to feed a million Dwarves and occupying a small fraction of the volume of a typical mountain.

Supposedly we could feed 13,300 people per square kilometer using hydroponics. Thus 75.187 square kilometers would be needed to feed 1,000,000 Dwarves. That would require three or four farming chambers 5 kilometers by 5 kilometers square, stacked one on top of another with 100 meters between each level for a total height of 300 or 400 meters.

Using aeroponics it might be possible to feed 49,210 persons per square kilometer, thus requiring 20.32 square kilometers. So 21 chambers one kilometer by one kilometer stacked on top of each other with 100 meters between levels should reach 2.1 kilometers tall and provide more than enough food for 1,000,000 Dwarves.

How many people can you feed per square-kilometer of farmland?1

But what about sunlight for growing crops inside a mountain?

The Fellowship of the Ring, Book Two, Chapter Four "A Journey in the Dark" has Gimli sing "The Song of Durin" about Durin the Deathless. The third stanza says:

A king he was on carven throne
In many-pillared halls of stone
With golden roof and silver floor
And runes of power on the door.
The light of sun and star and moon
In shining lanps of crystal hewn
Undimmed by cloud or shade of night
There shone for ever fair and bright.

Gimli seems to be singing that the ancient Dwarves had artificial lighting. With artificial lighting they could light their underground farms to grow food inside the mountains.

And the dwarves could have farmed land outside, in lands claimed by their kingdom. Maybe the Dwarves used a thousand square kilometers of outside farmland to feed a million Dwarves in Moria at its height. That would equal a square areas 31.62 by 31.62 kilometers, or a circular area with a radius of 17.84 kilometers and a diameter of 35.68 kilometers.

And when Dwarf economies were flourishing the Dwarves didn't grow much of their food supplies but traded their goods for food supplies from Elves and Men.

Thus the Dwarves didn't need to hollow out vast volumes under the mountains to grow food. Thus the underground cities of the Dwarves would need to be the size of Human cities and not the size of Human cities plus the farmland necessary to feed the populations of those cities.

The city of Philadelphia, Pennsylvania has a land area of 347.52 square kilometers and an estimated population of 1,567,872 and a density of 4,511.61 persons per square kilometer. Thus if Moria had 1,000,000 Dwarves and Philadelphia's population density it would have 211.65 square kilometers of floor space in it's chambers.

This equals a single level with a square 14.54 by 14.54 kilometers, or a hundred levels each 1.45 by 1.45 kilometers.

If the height of the chambers averaged 10 meters, the total volume of Moria's chambers might be 2.1165 cubic kilometers. And a typical mountain might have a volume on the order of 100 to 1,000 cubic kilometers, for example.

Thus the excavated volume of a Dwarf city in the novels should be only a tiny fraction of the volume of the mountain it is in.

The Dwarf and Orc underground realms in the movies are a different story.

In the movies it often looks like 90 percent of the volume of the mountain has been excavated for the underground city, a very unsafe thing to do. The mountain would probably collapse, crushing everyone in the underground city to death, long before it was excavated that much.

A better method to build an underground city that was mostly empty space instead of rock would be to tear down a Mountain A and transport all it's materials to another place and then build a building B that is the size of a mountain. And when the gigantic building is complete disguise the outside so that it looks sort of like a very regularly shaped mountain.

Of course in a giant, mountain sized building, the interior architecture and placement of supports would have to be very regular and symmetrical, and in the LOTR movies the interiors of Dwarf and Orc underground cities often seem very irregular and natural looking, which is very implausible.

And after seeing three questions in this series, I have the feeling that my answers about plausibility are going to be patterned something like yes, yes, no, yes, yes, no,...for the novels and no, no, yes, no, no, yes,... for the movies.

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    $\begingroup$ You have a REAL issue with the movies, don't you? Because that is what the series is primarily based upon. $\endgroup$ Dec 18, 2017 at 5:13
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    $\begingroup$ JohWDailey - I like the movies. In my opinion even the worst possible movies based on LOTR would be many times better than even the best possible movies based on most other novels, like, for example, Harold Robbins' The Carpetbaggers. But I don't like those aspects of the movies that are inferior to LOTR the novel, especially those that seem rather arbitrary and inexplicable. $\endgroup$ Dec 20, 2017 at 18:39
  • $\begingroup$ “In the movies it often looks like 90 percent of the volume of the mountain has been excavated for the underground city, a very unsafe thing to do.” We don't know how much bigger the mountain is. What matters, it seems to me, is that the vaults are implausibly wide. (Of course we never look up in the dark to see how they're made; do Dwarves know the gothic arch principle?) $\endgroup$ Sep 19, 2018 at 6:35
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    $\begingroup$ The movies are not bad per se. The issue is, in the books the setting is at least self-consistent, if not exactly scientifically accurate. A lot of small details are worked out meticiously. The movies on the other hand do not attempt such precision. Instead, they give us a sort of visual shorthand, heavily inspired by visual tropes of other fantasy settings - DnD and Warhammer, for example. There simplest example is Barad Dur, which should look like a huge squarish industrial plant according to the Tolkien's own drawings, but is instead some wh40k dark eldar pseudogothik spiky monstruosity. $\endgroup$
    – Cumehtar
    May 17, 2019 at 15:48
  • $\begingroup$ The large chasms shown in LoTR are really not that outrageous. Real caves systems such as Son Doong & Hang Son easily rival the soaring ceilings of the movies. $\endgroup$
    – Nosajimiki
    May 17, 2019 at 21:50

You don't need to do anything. You really don't. The visuals in LOTR and the Hobbit show some pretty huge underground spaces, sure. If you want to argue that the engineering involved is questionable, and just stonework couldn't support that kind of vaulting, sure, but the idea that what we see in the movies represents a significant hollowing of the entire mountain just doesn't parse, mathematically.

Here's Why.

Let's think about Manhattan for a moment. Around 1.7 million inhabitants plus all that office, manufacturing, entertainment, and transportation space. The whole thing is a bit less than 90 square km. Let's say we want to bury the entire thing under a mountain. You know, for Safety. converting into cubic volume is... tricky... I haven't found a reliable estimate yet, so I'm going to suggest an average height of 10 stories. Yes, a lot of Manhattan is a lot taller than that, but a lot more of it isn't. I'm going to go with it. It doesn't really matter though, as you'll see in a minute.

10 stories = ~30m = .03km x 90square km = 2.7 cubic km.

Now let's compare that to a mountain.

The largest mountain on our planet is Mauna Loa in Hawaii. Measuring from the sea floor it's more than 4km tall and has an estimated volume of 75 cubic km.

Let's cram Manhattan in there.

2.7cubic km (Manhattan) / 75 cubic km (Mauna Loa) = .036% of the volume of the mountain.

Well, sure, ok, but Mauna Loa is a VOLCANO. That's not safe.

Fine. let's do Everest. Everest is a measly 60 cubic km by comparison, so Manhattan would take up a more respectable .05% of the mountain.

That leaves an awful lot of free space... maybe we can put all of New York City in there?

Sure, why not.

The New York City Metro Area takes up about 775 square km. Again, an average height is pretty difficult but since most of that 775 square km is suburbs it's probably going to be a lot closer to 2 or 3 stories. Let's go with that.

3 stories = ~10m = .01km x 775square km = 7.75 cubic km.

Stuffing that into Everest gives us 7.75 cubic km into 60 cubic km = 12.9%.

So, TLDR, even putting one of the largest human cities on the planet entirely underneath a mountain only takes up a bit more than 10% its volume.

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    $\begingroup$ Points for the math. It looks big and imposing, but the reality is that when you move from a largely two-dimensional paradigm (see: towns outside the major 5-million-people skyscraper-filled mega-cities) to a three-dimensional one, available space increases exponentially. Khazad-Dum is probably a lot smaller than said mega-cities overall, as all of Gondor could not summon an army of a hundred thousand even with levies: there would not have been anywhere near a million Dwarves in Khazad-Dum even at its height. Essentially, it's like occupying one room in a 100-story skyscraper: negligible. $\endgroup$
    – Palarran
    May 16, 2019 at 23:52
  1. In the Silmarillion, Tolkien indicated that the mountains were created rich with gold and gems easily accessible to the dwarves. They awoke, the Silmarillion says, from some sort of slumber in the mountains. My take on that has been that the dwarven kingdoms were more settled than mined. The vast reaches were available because Eru made them that way.

  2. Again, Eru (who made everything) made these caves for the dwarves to live in. Dwarves woke up, so the Silmarillion goes, in these caves. They could grow mushrooms without light. Sinkholes from above could bring in wood and biomass to fertilize the mushroom crops. Underground rivers and blue holes (underwater sinkholes) could route fish into the kingdom.

  3. The visualization of all the torches often makes me want to ask, 'where is the air?'. But it is a world of magic - maybe it is a magical fire that gives off light and heat without consuming fuel or oxygen.

  4. This is the dwarves natural terrain. It was, no joke, made for them and them for it. Maybe vertigo is just not something dwarves have when they feel the safety of a few million tons of rock over their heads.

  • $\begingroup$ I asked for Reality-Check. $\endgroup$ Dec 18, 2017 at 2:41
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    $\begingroup$ Saying, "Magic was used in Tolkien's work," does not answer the question of how to apply this to a physics-constrained world. $\endgroup$
    – Frostfyre
    Dec 18, 2017 at 6:07
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    $\begingroup$ Maybe I don't understand how we use reality-check here. There may be a FAQ on it, but I understood it to mean 'can this make sense in any reality?'. I checked, and this is the description for the tag : 'realistic in a given context. Answers should say yes or no, with supporting info. Contrast with: science-based and hard-science tags. This tag ' Hard-science may have been the tag you are looking for. The answer provided is an answer from Tolkien's context. $\endgroup$ Dec 19, 2017 at 15:27
  • $\begingroup$ Air is easy, large open spaces with high windows on a mountain will generate their own currents and circulation due to wind and thermal gradients $\endgroup$
    – John
    May 16, 2019 at 20:27

Regarding circulation, there is an easy solution. In some medieval mines, people used large furnaces situated beneath chimneys to generate an air current. It would have been relatively easy for Dwarves to do something similar. Air would flow into Moria through holes at the base of the mountain, and out through the many chimneys.

However, I don’t believe anyone in the books describes the dwarven mountains as smoking.

Another potential inspiration is the ventilation systems employed by Termites: the top of termite mounds are porous, allowing fresh air from above to flow in, and hot, stale air to flow out. This keeps the air in the top of the structure fresh. Of course, termites don't live in the top of the structure, so this is only a part of the system.

Going into more detail, termite mounds are built around a single, central chimney, surrounded by porous walls with many cavities; these cavities, as well as the central chimney, go all the way down to the base of the structure. In the day, the air in the surrounding cavities heats up more quickly than that in the central chimney, due to being closer to the surface. As a result, that air rises, and the resulting pressure difference forces the air in the chimney to sink down into the base of the structure.

At night, the air current reverses: the air in the outside system cools more quickly than that in the central chimney, so it sinks while the chimney's air rises. The result of this is that no matter the time of day, Termites can breathe even when deep in their own mountain fortresses... and not only that, but the temperature in their nest is relatively stable as well.


We, unfortunately, never have enough information about the Dwarves in Tolkien's books. We mostly see them from the outside points of view - either elven in Silmarillion drafts, or hobbit one in the Hobbit and LOTR.

There is some information in those books, however, that lets answer some of your questions.

First off, I do not think dwarves would just hollow out a solid mountain. Wherever we have mentions of underground structures, they are based on some natural caverns. Nargothrond was built by the elves in the natural caverns that were fist inhabited by the dwarves. Menegroth, it seems, was also build by dwarves for the elves on the basis of the natural cave. As far as I remember, the river Celduin flowed from inside the Lonely mountain - I always assumed it looks something like Lod Cave in Thailand before the dwarves repurposed it into a city.

There is also a telling passage in the LOTR, where Gimli speaks about the caves of Aglarond and how the dwarves would work on transforming these natural caves to a dwarven city if he had his way.

Overall, the idea of a fortified underground 'burh' is a pretty popular one in the Middle-Earth, for both elves and dwarves. The halls of the Elvenking in Mirkwood are in fact a lesser version of Menegroth - a fortified underground dwelling that serves as a kingly castle, with most of the population actually living in the forest outside.

Another staple of an underground sinda-dwarven 'burh' is an underground river flowing from it. It would take care of water supply and, possibly, sanitation. Moria has Silverlode flowing from it, Forest River flows through Elvenking Halls, Carnen flows from the Iron hills, Celduin goes from the Erebor. Nargothrond was in the caves on the bank of Narog. Esgalduin, if it doesn't flow through Menegroth, is extremely near. (I do not actually remember how the relationship of the city and the river was describe. It seems to me that the entrance was across the bridge over the river, but I may be mistaken).

Speaking about the food supply, we actually never encounter the idea of a completely enclosed self-sufficient underground dwelling, neither for elves nor for dwarves. The only completely self-sufficient city without any links to the outside was Gondolin, and it was not in a cave system, but in the open space surrounded by the mountains. All other underground burhs - both elven and dwarven - had their gates open most of the time. For example, I remember Moria closing it's western gate in the Second age, but it seems their eastern entrance stayed open all the time.

The dietary requirements of the dwarves seem to be the same as the other people of Middle-Earth - they eat their food for extended periods of time, seem to like it and never display a need to eat some particular product to refill a microelement deficiency. (Actually, elves, humans, hobbits and orcs are biologically the same species according to Tolkien himself. Dwarves are a close copy of the same design by the other maker, so it seems they are still a carbon-based lifeform, although not genetically compatible with everyone else.)

So it seems to me it's not a stretch to assume that dwarves actually farmed and raised animals (say sheep and ponies) outside of their cave cities. Unless they could find a settlement of other people nearby and establish trade relationship - Erebor and Dale, for example, had it all worked out perfectly before the dragon appeared.

As for the light and air - we all remember the description of the air shafts in Moria. Presumably, dwarves were good enough engineers to calculate the neccessary number of shafts and their ideal locations to ensure the proper air supply. Having big enough openings for underground river flow in and/or out, as Tolkien's underground cities often have, would also help with air circulation. We also have mentions of some sort of artifical lighting in the song of Durin, as aready discussed before.

As for agoraphobia and claustrophobia. I think the movie indeed does overdo the 'gaps' a lot. Mostly the dwarven cities would be differntly sized caves, connected by artificial corridors. All the dangerous gaps would be spanned by bridges, separated by railings, etc. Now, claustrophobic dwarves would have it bad, though.

  • $\begingroup$ I'm pretty sure Aule made sure there was no such thing as a 'claustrophobic dwarf'. Otherwise, really nice writeup! $\endgroup$ May 17, 2019 at 14:22
  • $\begingroup$ That's a more complicated question then may seem. Tolkien's relationship with sciences was shaky, and his metaphysics changed a lot during his life. The latest drafts of Silmarillion had some attempts to bring the story in line with the science as he knew it. There was some weird stuff stemming from this - like Arda was always round, Morgoth's moonbase, orbital bombardment, Numenorian steamships, etc. So, it's not easy to understand how heredity and genetics works in Middle Earth, and what influence does 'spiritual' stuff have on it. $\endgroup$
    – Cumehtar
    May 17, 2019 at 15:04
  • $\begingroup$ Judging from the existence of the Petty-dwarves, there is a possibility of hereditary genetic change, unless it was purely morphological from systematic lack of food. $\endgroup$
    – Cumehtar
    May 17, 2019 at 15:05

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