Realistic underground shelters for long time hibernation for mainly humans

Question

Short variant of question:

1. I have few big natural caves (really big).
2. I want to transform it into shelters capable of keeping ~2.000.000 humans in hibernation, sleep, suspended animation or other thing - so they can sleep for 5000-10.000 years and wake up on one day being mainly healthy and sane (and being THE SAME person they descended to sleep).
3. This system does not need to be fully automatic - we can wake up some members for maintenance of sleeping ones' live support system and performing cave entrance guard duties.
4. Ideally, it can all be done with 20th-21th century technologies (we assume we have few biology breakthroughs happened earlier in 19th century).
5. Shelter inhabitants are not 100% humans, they have more skills in biology than us, and can alter themselves a little before going to sleep.
6. We need at least 80% percent of sleeping population survive in 5000 years and at least 60% survive in 10000 years.
7. There is bronze age population evolving on surface, we need to keep if away from our shelters.

The question is - what breakthroughs in biology do we have to achieve in the 19th century for this shelters be build-able in the 21th century?

Long variant of question:

In my story there is a human nation. Lets call them Forest Confederation.

They are on technology level of early 20th century (but with a few additional breakthroughs in biology) - their analogue of the First World War just finished recently and they are victorious - they used quite clumsy, by our standards, aeroplanes loaded with bacteriological based weapon spreading airborne.

Of course the people of the Forest Confederates have all undergone proper vaccination, but not their rivals - they have lost 90% population turned into short living turbozombies killing the remaining 9%, and sparing the most lucky 1 percent. In long time this 1% can recover at least to Medieval level.

Unfortunately, the Forest Confederation has no choice but to perform this forced genocide, because their more industrial advanced rival would give them no chances.

Also killing 99% of the population granted attention of a nearly omnipotent Mother Nature Goddess (the Forest Confederation has a strong faith).

Or at least the Forest Confederation interpreted a few dark omens as occurences by the will of their goddess.

So, the will of the goddess is clear - "I will perform major climate change in 200 years from now and all of you will either die or degrade back to stone ages."

The Forest Confederation treated this like real danger and started building underground shelters to survive the climate change. I have posted the requirements to this shelters in the short version of the question above.

I have read this question, but it has 23th century technology level and I want something more simple.

I do not want You might want to consider transporting digitised copies of the people, and then 3D printing them new bodies at the other end. as pointed by Jnani Jenny Hale (https://worldbuilding.stackexchange.com/a/66404/2763),

I want something more close to Zxyrra's answer (https://worldbuilding.stackexchange.com/a/66452/2763) with tech level close to present day, maybe with some handwavium.

I want the Forest Confederates leaving their shelter after 10.000 years being the same ones who entered it.

UPD 1: power is not such a drastical problem - we can receive power using geothermal energy sources even using 20th century technology.

Answer 1

If they aren't human just make their reproduction system different? Creature has capacity for sexual reproduction, but not necessity.

Sexual reproduction creates a conjunction of the participating parents, a new being. Asexual reproduction essentially clones the existing material of the parent. You could include with this process a change in neural formation in the organism that creates a more definite & closely defined template than that of humans to the point where their personality is essentially identical..

Or instead of that some 'handwavium' neuronal transfer so you're effectively the same person, but not actually the same body. Treat the memories of the species as a virus/prion, sorta.

If you want them to be biologically human in ancestry at least, use some epigenetic effect of the local environment or..like bacta tanks in star wars.. it's not a technological discovery, but a use of a natural resource.

Actually making an underground settlement is pretty simple.

People need: Heat regulation & Regular provision of certain chemicals & compounds.

So you need..heat source, heat exchangers, water source, organisms that live underground whose products or byproducts are useful. upper chambers could be filled with a fungus/bacteria/tree roots that convert co2, there's nothing banning inland caves from having subterranean access to water tables or even the sea. bred or natural organisms that can filter saline, no problem?

Thirdly, surround all your cave entrances with something that's extremely poisonous to whatever this bronze age species is (human also?) well, learning how to and having the will to remove said poisonous flora could well take 10,000 yrs if the species is comparatively retarded.

Bear in mind how long the move from bronze age to powered flight took humanity..and probably nothing your 19th/20th century civ is going to do is going to last 10,000 yrs and remain effective, your poisonous organism might die from drought or evolve to be less poisonous or bronze aged peoples find it's particularly useful for poisoning other bronze age peoples and start harvesting it with slaves for use in war or w/e.

Answer 2

If you want to build a structure to last for 10 000 years you really have only one option: pile rocks on top of rocks the way they did in Ancient Egypt; essentially, build a small hill. Nothing else will last that long. Build it in a hot desert -- temperate or subarctic latitudes are a no-no, especially with water around. Don't have anything electronic in it; no way the materials used in electronic circuits will last for 1000 years, let alone 10 000.

In short, the millions of hibernating humans will never get a chance to wake up.

Alternatively, don't build the facility to last 10 000 years. Instead, built a small country, complete with farms, factories, schools, universities, hospitals and an army. Task the country with the maintenance of the cryofacility. A population of a few million may be sufficient; say 10 million. In favorable conditions (good climate, good agricultural land, sufficient rain but not too much, access to the sea and natural resources such as iron and copper and tin and zinc and gold and petroleum), a country of 10 million people does not need more than 50–60 000 km², which is about 250×250 km or 150×150 miles. Small enough to make it plausible that the rest of the world can just ignore it; for example, the island of Madagascar is 500 000 km² and is very inaccessible for people lacking modern technology—in the real history it was the last large landmass to be populated by humans, and that did not happen before the 3rd century BCE (or CE, opinions vary) and the human population remained very low until the 6th century CE.

How to make those millions of people to (1) not multiply to excess and (2) stay focused on the mission for 400 generations is another problem. Please note that the entire history, from the very first cuneiform tablets in Sumer to this day, is about 5000 years: your intention is to make a facility and support structures lasting for two times the duration of the entire history.

Answer 3

There's no way that anything more complex than a nail created with early 20th century technology would survive 100 years of continuous work, much less 5,000 or 10,000. Even if you have some people awake to make maintenance, they'll run out of spare parts and replacements quickly, not to mention power sources or food and waste disposal for the awaken.

Answer 4

The keeping of a specimen alive and as it is longer than its lifespan should be the main issue. That can be solved by:

• Self-replication: Making copies of oneself to survive longer (asexual reproduction as already suggested)
• Negate Aging: By means of cryogenization or suspended animation (if technology allows)

If we can get a source of water and convert whatever power we get (geothermic for example) into something processable by the species, sustaining life wouldn't be a problem either.