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A massive empire has decided, for cultural reasons, to build a massive, massive artificial cave chamber underground. They're willing to set aside 75 years for this project (about thrice the time it took to construct the Great Pyramid of Giza). They possess technology and understanding of science and mathematics slightly surpassing both the Roman Empire and the countries of the High Medieval Period. Manpower is almost a non-concern.

They wish to 'construct' it by carving and removing rock, not by adding support/buildings. It should be 'free standing,' meaning no support columns should be left in place. It should be more or less domed shaped, with the sides touching the floor being practically vertical before they begin to curve, and being about (flexible on this point) half as high as it is wide. Building off existing natural formations is perfectly acceptable, and a natural formation up to as large as the Sarawak Chamber is acceptable. The room shouldn't be submerged (at least once the project is finished).

The geography and geological formations (in both nature and variety) are roughly comparable to that of the German Empire. The cave room should be safe for centuries to come from threats of possible earthquakes or any surface concerns. It doesn't need to survive any Earthquake, but it can't be teetering on the edge of instability.

How big could this cave room be, given these constraints? Are the resources (manpower, time), knowledge (of the period), or materials (the material being quarried into) the limiting factor?

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    $\begingroup$ Material(s) would definitely be a factor. Soft stones are easier to remove, but not typically as sturdy/stable. Also, half as high as it is wide with no columns definitely could be tough in the center. Half as wide as it is tall would be MUCH more stable. Though if they have a good grasp of science and math, what would their reason be for NOT leaving support columns? Because, you know... physics. $\endgroup$ Oct 4 '16 at 16:15
  • $\begingroup$ @JesseWilliams I was trying to pick what to leave in/take out for the question, but the cultural reason for no support columns is that they're trying to create a 'dome' metaphoric representation of the night sky. This chamber will ultimately be used as a memorial (sort of a tomb, except they don't actually store the dead there, and it's not sealed off). $\endgroup$
    – Ranger
    Oct 4 '16 at 16:18
  • $\begingroup$ Hmmm, I suppose that's a decent enough reason. But they'd have to know that makes it tougher. I would say so long as the dome was reasonable higher than it was wide, and was a proper dome, even weaker materials should suffice, so long as they aren't brittle materials. Something like shale would be want to crumble over time. $\endgroup$ Oct 4 '16 at 16:23
  • $\begingroup$ @JesseWilliams I'm actually flexible on the exact proportions, as long as it looks dome-like to somebody standing in the center, and I imagine that the height could be pushed up without distorting the appearance too much. $\endgroup$
    – Ranger
    Oct 4 '16 at 16:25
  • $\begingroup$ I thought this would be an easy answer one, but the needed data isn't available it seems. There are some tools that romans used that you could test to see their mineing rate and just assume multiple teams working around the clock for 75 years that would give your answer, but I don't see any info online for that so unless you personally build and test it, it's a no go. And looking up mines is a no go too, but there are some massively large mines even in the neolithic so just about anything you decide will not only be possible, but plausible. $\endgroup$
    – Durakken
    Oct 4 '16 at 16:44
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How big could this cave room be, given these constraints?

Romans seem to have dugs mines in the size of 2.2 km long before stopping and 200 meters in depth, often just stopping at the water table. There seems to be no reason to suspect they couldn't build a big dome shaped cave underground of any size given their other tech achievements and the scale we see here.

The size of rocks often shut down a mine, but that's because they were hoisting things up a deep shaft, they probably thought the mine wasn't worth any more time, etc, because they certainly could have broken a rock down if they really wanted to.

Besides this, to get a dome without a hole at the top you'll be using either a straight horizontal shaft or a vertical shaft down then horizontal shaft in. Either way this really isn't a limit to people who really want to do this, but to make it more time plausible let's say we go the horizontal route and that means that any cave they make can only be as big as the mountain they can get to, no bigger. That's massive, not helpful, and you're limited to where you can put it, and it's easier to spot with lower technology.

Let's say you wan to build underground rather than undermountain... Well basically, because we know that the water table and shaft depth is around 200 meters the answer to that is a ~200-400 meter diameter dome, based on their biggest dome they ever made (The Pantheon) which is 43.3 meters in diameters and height, meaning dome walls are very thick, but because we're already starting with rock material they should be able to make it any size based on the same principles as long as the material is dense enough...

So the answer in just size is...

  • Limited to what we've seen: 43.3m
  • Extrapolated from what we've seen underground: 200-400m
  • Extrapolated from what we've seen undermountain: as big as the mountain.

Are the resources (manpower, time),

Here's the thing. We know that Roman soldiers could build cities very rapidly and certainly there was enough soldiers/etc. The issue is that they never undertook these projects, they were funded privately and really slowly with not the greatest labor. So when they built the Pantheon they likely didn't have a lot of labor nor quality labor nor was it a thing to get done right away... plus there is a difference between building a dome and cutting out stones to form one.

The Pantheon took some time less than 110 years to build. The inception of building was in the 30s BCE and it was partially destroyed in the 80s CE. so it had to have been finished some time in that period.

Assuming I'm right about how slow they were going and how fast they could go, my bet is it wouldn't take them that long to actually build any size. More over, we're dealing with just the removing part of the stone. We're dealing with roughly double the size of the pyramids with about the same amount of work, so I would say 20 to 40 years would be quite plausible for the underground 200-400 meter diameter dome.

If you build this farther out it limits manpower, closer in more man power. And you can just use the Chinese and Egyptian way of getting workers... it's a tax right off.

The biggest issues that you're going to face are...

  • Ventilation
  • heat
  • light (not realy since you can use candles)
  • getting man power in the cave at first
  • getting rock out.

Once it is built you're going to have to deal with as time moves on things get built and a cave under a city will eventually get dug into if noone knows its there. If you build it further out you solve that problem for a while but then lose man power.

Without more in depth knowledge of their mining abilities and architectural abilities, that's the extent, I think, of what a layman can say about this question...

Source for Roman mining info : Here

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For reference the largest natural cave:

Miao Room, measuring 380.7 million cubic feet in volume.

Borneo's Sarawak Chamber, has a larger surface area than the Miao. It measures 1.66 million square feet, is nearly 2,000 feet long and 260+ feet high - can hold 40 Boeing 747s. Deer Chamber, ... is so big that it could fit five cathedrals.

Son Doong has caverns large enough to hold a 40-story skyscraper. Collapsed roof allows sunlight in some places, where 100' tall trees are inside the bottom of a cavern. Son Doong was unexplored for a long time (1991-2009) because of the huge drop the entrance required. The caverns are said to dwarf visitors with their immensity, and that the silence was eerie for first visitors down the hole into the huge cavern. Interior weather systems, with cloud formation (water and air exchange), cool air into hot jungle above makes mist, too.

Perhaps builders merely excavate light holes and mirrors/lenses for making a non-collapsed natural cave into a night sky during the day, and complete darkness during the night.

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