system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: Fallout Vault or any Underground shelter can't house 100 people for a prolonged period of time.

Just to preface, Fallout Shelters are the only underground bunkers that I have knowledge on. I will be using the general concept and what I have observed in-game (Fallout series) to argue.

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Food: Realistically, there isn't going to be enough space to store the amount of food needed to feed one hundred people for years. Even if we are taking in Hydroponics to grow food indoors, you would need the supplies, water, and the space to grow ample amount of food to feed that many people.

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Water:  Getting clean water is going to be a challenge. In the Fallout series, the vault had some special technology to have purified water, but the chips aren't easy to come by. In the real world, I don't even see how we can get clean water in a post apocalyptic scenario.

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Supplies: Stuff likes clothes and other necessary materials  will be extremely scarce. The bunkers/vaults would need parts for maintenance and there wouldn't be any feasible ways to produce these materials without factories.
gpt
> Food

Some quick calculations here. The average American (i.e. well above bare minimum) consumes 1000kg of food a year. Suppose we take this as potatoes (an underestimation, since potatoes are quite energy-dense). Potato yields per year on normal soil are anywhere from 12 tons to 30 tons per acre. Suppose that we take 25 (again, an underestimation because vault conditions allow for ideal conditions), then we need four acres, or 16000 square metres of space for 100 people. Hydroponics has anywhere from 5 to 10 times the yield of soil-based crops, so that cuts things down to 3200m2 to 1600m2. That's quite achievable.

> Water

Building the vault at the right place avoids this. Wells, especially ones tapping into deep, isolated aquifers, can provide a lot. Recycling water is also a must.

> Supplies

This is an absolute non issue. Clothes last a long time, especially if you build them for durability. Adopt a less custom-fit size system, and you can manage this very easily.

Multiple redundancies would make having spare parts a minor issue at best. Having a blacksmith in the group renders tools a non-issue, since you can recycle almost everything. We have built much more failure sensitive habitats than a vault already.