Short variant of question:
- I have few big natural caves (really big).
- 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).
- 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.
- Ideally, it can all be done with 20th-21th century technologies (we assume we have few biology breakthroughs happened earlier in 19th century).
- Shelter inhabitants are not 100% humans, they have more skills in biology than us, and can alter themselves a little before going to sleep.
- We need at least 80% percent of sleeping population survive in 5000 years and at least 60% survive in 10000 years.
- 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.