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Liquid water at that height is going to be a challenge: at the Equator the limit for perennial snows is 5000 meter above sea level, so you see that there will be no liquid water in the open.

You could go around this if you have some sort of heat source, which can provide local warmer conditions. You mention an extinct volcano, so it might be possible to have some geothermal water source, creating warm oasis in an otherwise barren and frozen land.

The lack of a sufficiently dense atmosphere is the second hurdle, both for plants and animals (humans included). At that height you are at the border of the death zone due to the severe scarcity of oxygen, therefore the only solution I see as viable is to have some sort of sealed cave system where a higher atmospheric pressure can be maintained. Gases might be supplied by volcanic activity, mostly CO2 which is then converted in O2 by the plants.

But a cave has a lack of light. You might handwave a bit around this, giving a ice dome sealing the volume you are interested. The ice being transparent would let some light in, and would seal the gases from a quick escape to the outside. Volcanic heat could explain why the ice has a dome shape.

If Reinhold Messner can climb all 14 peeks over 8,000 without oxygen, and do Everest twice in two years without it, then a civilisation can happen at that height.

That is false. Messner wasn't depending on running water or forageable food at those heights: he was bringing it all with him. He wasn't staying any appreciable time at those altitudes, either. There's an astounding difference between one incredibly well prepared guy momentarily scaling all those heights and a community enjoying day-to-day existence at those heights.

Please read this wiki article. The highest permanent human settlement is 5,100 m and the tolerance is 5,950.

Therefore, we already have a problem. Your Earth-like planet... isn't.

• There's not enough atmosphere (especially oxygen and carbon-dioxide, which plants need) at those altitudes. The atmosphere would need to be thickened.




• The tree line is approximately 3,050 meters (10,000 ft). The highest altitude moss (thought to be the highest altitude plant life) grows at 6,480 meters (21,260 ft). Animals come close, the Yak (with two coats of fur!) can make it up to 6,100 m (20,000 ft). But nothing lives at 25,000 ft without help.

What, then, can be done?

1. You can increase the density or mass of the planet such that it retains a larger atmosphere. Gravity decreases as you increase altitude... but not that much. So you'll likely have a high-G race. You're also going to have a higher percentage of low-altitude deserts as a thicker atmosphere will increase sea-level temperatures. This is the only natural solution I can think of and the consequences are very high (which is to be expected). Note also (thanks to Arkenstein XII), that as the mass of a planet increases, the tendency is for mountains to be lower, reducing the plausibility of the scenario.




2. You can bring man-made structures into play: plumbing low-altitude atmosphere to your settlement. You'd be moving a LOT of air, but it wouldn't necessarily create wind (just volume). It would dissipate quickly, meaning your settlement would have defined limits, and if the surrounding climate pushed the wind speed up, it would be easily blown away. You could do the same for water with heating at the top of the pipe. However, none of this would allow plants to grow — too cold.




3. Underground, in a dome, sealed away from the environment (L.Dutch's answer).




4. Seriously change your race such that it's not human anymore. Capable of living at very low atmospheric pressures (would die at sea level), capable of horrendously high UV (might even need it for photosynthesis), capable of living off of lichen and eating snow. Very happy at very cold temperatures (like the Yak, maybe two coats of fur... again, would die at sea level). Etc.

The more earth-like your planet, the less possible your goal.

The highest settlement on Earth is supposed to be at 16,830 feet or 5,130 meters, which is only about 0.6732 of your altitude of 25,000 feet or 7620 meters. https://en.wikipedia.org/wiki/List_of_highest_cities1

I doubt that there could be a human settlement at 25,000 feet, 1.48 times the record, on Earth without advanced technology like in an Antarctic base or a Moon base. Thus conditions on your Earth-like planet should have to be less than 100 percent Earth-like to make it possible.

It would help if that Earth-like planet has an atmosphere significantly denser than Earth's. Thus it is noticeably denser at any altitude than Earth's atmosphere is at the same altitude.

It would also help if the Earth-like planet had a weaker surface gravity than Earth. Thus it would pull down and compress the atmosphere a bit less and the atmosphere would thin out a bit less with increasing altitude.

As a general rule, astronomical objects with lower surface gravity and lower escape velocity are expected to have less dense atmospheres, not more dense atmospheres.

But exceptions are possible, and in fact there are two exceptions in our solar system. Venus has slightly lower surface gravity and escape velocity than Earth, but for various reasons has an atmosphere about 90 times as dense as Earth's. Titan has much lower surface gravity and escape velocity than Earth, but has an atmosphere a bit more dense than Earth's.

Another factor which might help would be to Have a higher proportion of oxygen in the atmosphere of this Earth-like planet, so that the thin air in the high valley will have a bit more oxygen in it than Earth's atmosphere at the same overall pressure.

So I imagine your Earth-like planet might have a little more than 90 percent of the surface gravity of Earth, and a little less than 110 percent of the atmospheric density of Earth, and a little less than 110 percent of the percentage of oxygen in the atmosphere, and that might be enough to have sufficient atmosphere and oxygen at a height of 25,000 feet or .,620 meters above the sea level of that world.

But you might want to have someone calculate the atmospheric conditions at 25,000 feet in such a world to make sure that would be enough.

And any Earth people who might be in the story might describe the planet as Earth-like, with less than a ten percent difference from Earth conditions.

In most parts of the planet the different conditions will have little effect on the various ecosystems and the differences will only be significant at extreme high altitudes.

I believe this is not possible for a pre-industrial society. The highest permanently inhabited 'settlement' is indeed at around 5,200 m. But from my experiences in the Himalayas (husband of a doctor who worked at high altitude settlements there), the highest altitude villages are usually considered to be seasonal and the inhabitants need to descend to below around 4,500 m at intervals, in order to 'stay healthy'. Furthermore, in such communities, pregnant women almost always drop to still lower altitudes (from memory below around 4000 m) to give birth as babies do not 'thrive' above that altitude.

I guess that over a very long time period people could evolve/adapt to those conditions. But after several thousand years of adaptation, populations in the Himalayas and Andes are still more than 200 m below your target altitude.

what conditions are needed to support a human community?

Bearing in mind the criteria that you stipulate.

• a stable community of completely isolated pre-industrial humans


• They live at an altitude of (at least) 25,000 feet (7620 meters)

The conditions needed will be at least one of the following three options.

1. A lower altitude or higher atmospheric density so they're effectively living at lower altitude.




2. A significantly higher knowledge of & ability with scientific principles than would normally be associated with a pre-industrial society, to provide artificial conditions to live & grow food in.




3. To not be isolated from lower altitude settlements that can provide them with food & other supplies, because nothing grows naturally at that altitude.

So in short, you're going to have to give a little on at least one of your chosen criteria.

If you use option 3 you need a reason other communities supply them, if there's no commodity found exclusively in the mountains they can trade for supplies some sort of religious community like Buddhist monks might fit the bill, but I suspect isolation is the last criteria you want to surrender.

Perhaps the easiest way around things is to say that they live in a valley at a comparable altitude to the highest known permanent human settlements but that the only access to it is through a pass (or route) that climbs to the altitude you want.

Some significant altitudes & the effects of altitude on humans.

Armstrong limit altitude 60,000 to 62,000 feet.

Death zone altitude 26,247 feet.

The altitude you want 25,000 feet.

World’s highest plants 20,177 feet.

Highest altitude humans can survive 19,520 feet.

Highest permanent settlement in the world 16,700 feet.

Snow line altitude 15,000 feet.

Organisms at high altitude The Himalayan pika lives at altitudes up to 13,800 feet.

Treeline altitude 10,000 to 13,000 feet.

Effects of high altitude on humans

Taking those details into account perhaps the highest plausible altitude for your community is 19,520 feet & they will need considerable acreage per person to sustain themselves on plants that can grow at that altitude as those plants grow extremely slowly (in part due to the cold), that they might have domesticated Yak & Pika adapted to the altitude for meat, wool & milk etc seems reasonable.

So in summary, perhaps set your community in a high valley or plateau in the mountains with an average altitude of around 19,000 feet that can only be accessed from high passes through the mountains with altitudes of around 25,000 feet at their highest points?

At that altitude the things that are going to form the base of your food chain are lichens, mosses & cushion plants, nothing else grows at that height & you'll have no trees for wood.

This is Lobuche. It's located at 16,207' in elevation in the Khumbu region of Nepal. It's the last stop before the Kumbu glacier on your way to Everest Base Camp.

As you can see, there is no real vegetation. Moss grows here. Some small ground covering plants. Most plants stop growing around 15,000' rocky tundra begins to give way to snow fields. Moss will keep growing up to 21,000', but that's the upper limit of where you will find vegetation.

Some mammals like yaks can be found up to 20,000'. In fact, they don't do well below 10,000'. Snow leopards live between 10,000-17,000'. Rodents like the Pika do not generally go above 14,000'.

Nepalese and Tibetians live and thrive at around 15,000' and it's possible to be born, live and die in this area and never go lower. However, it should be mentioned that they have a genetic mutation that allows them to thrive at high altitude that most of humanity lacks. Oxygen is only 60% of what you would find at sea level. Villages like Naamche Bazaar are at 11,000' look like any remote village. You could live here after acclimatizing and carry on with a normal, if not treeless life.

The biggest problem with your group is the elevation. At 25,000' they are clearly above the high altitude habitats where humans can survive.

The biggest problem is that oxygen levels drop as you rise in altitude. At sea level, the usable oxygen is about 20%. At Naamche Bazaar, the effective oxygen level is 13.7%. At Lobouche, it drops to 11.4% and at 25,000' where your people will be living, the oxygen level is at 8%. You're at the upper limits of extreme altitude, above food sources and in a zone where long-term survival is pretty remote because of the caloric and water intake needed to sustain your current weight. You need a lot of calories to maintain the body. There is also an issue with altitude sickness that starts with people above 8,000'. Later in life, Sir Edmund Hillary, the first documented human to climb Everest, damaged his health and was unable to go above 11,000' without becoming very ill.

I actually talked with him about this very subject on a ridge overlooking Naamche Bazaar many years ago. He was there dedicating a school and we attempted to get him to come with us to Lobuche. His spirit was willing, but he was already at the upper limits of where he could function without getting altitude sickness.

It's remotely possible for your group to exist at 25,000', but to make this story realistic, you are going to need a solution to food, water and how they maintain pretty good health. One thing is for sure, few people lowland are going to bother them.

Good luck.

• https://en.wikipedia.org/wiki/Organisms_at_high_altitude


• https://en.wikipedia.org/wiki/High-altitude_adaptation_in_humans


• https://www.higherpeak.com/altitudechart.html