A large solar flare event ignites the atmosphere of Earth, but luckily, humans have been preparing for an event like this for some time. We have a fairly large shelter around 2.2 kilometers underground.

Energy is no problem, as we have built heat wells where they bring up heat into a controlled area, and we then run water over a heated cathode to spin a steam turbine.

Assumptions about the environment and living conditions:

  1. No materials can be retrieved from surface.
  2. They are alone in their venture to survive: no divine intervention, no magic.
  3. They need a sizable amount of water for their needs, and for vertical hydroponic farming.
  4. The shelter accommodates up to 1000 people at a time. Assuming that each person drinks half a gallon a day, that will average out to 500 gallons for people, plus that for the farm.

I would prefer for the colonists to have a liberal amount: enough for baths or other luxuries.

How do I explain how these survivors get water?

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    $\begingroup$ What about artesian water? It should be sealed off from the surface in the type of event you describe. You'll have to drill sideways to get it, and you can find where it is beforehand if the catastrophe is anticipated. Also, you will have a lot of waste water to be recycled. $\endgroup$ – Dallaylaen Nov 22 '15 at 14:03
  • $\begingroup$ @Dallaylaen Great idea! $\endgroup$ – Quiquȅ Nov 22 '15 at 14:07
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    $\begingroup$ A solar flare can't "ignite the atmosphere of Earth"; it's not a combustible mixture of gases. If it was, then of course it would already have been ignited by lightning, volcanoes, etc. $\endgroup$ – Mike Scott Nov 22 '15 at 17:52
  • $\begingroup$ If bad comes to worse, burning hydrocarbons for steam. Oxygen is obtainable from many rocks, hydrogen is not nearly as common but it does occur in mineral oils; CO2 + H2O, what the plants need... plus a lot of energy which you will need to provide light for the plants anyway. $\endgroup$ – SF. Nov 23 '15 at 13:32
  • $\begingroup$ This is essentially a closed system, wastewater will be recycled, moisture from the air can be captured with dehumidifier, so once you charge the system, there will be very little need for additional water. $\endgroup$ – ventsyv Feb 17 '16 at 21:49

Given that they're 2.2 kilometers underground, it looks like they can take advantage of the water table, the place where rocks are saturated with stored groundwater. The water table often holds aquifers, which can be accessed with only a bit of drilling.

Is groundwater accessible 2.2 kilometers down? Well, they're certainly below the water table. I've found a map of the water table depth of Wisconsin (chosen at random), and it shows that the water table is seldom more than 50 feet below the surface:

It is important to note that the water table doesn't mark the area where the water is stored; it simple marks the highest point of stored groundwater.

We do have a problem, though: How deep does groundwater go? 2.2 kilometers is pretty far down, relative to some sources of groundwater. For a good survey on aquifer depth, I chose this report (Ashworth & Hopkins (1995)), a summary of various studies of aquifers in Texas. They identified nine major aquifers in the state. Here they are, along with the estimated water depth.

  • Ogallala: 600 feet
  • Gulf Coast: 3,200 feet
  • Edwards: 600 feet
  • Carrizo-Wilcox: 3,000 feet
  • Trinity: 900 feet
  • Edwards-Trinity: 800 feet
  • Seymour: 360 feet
  • Hueco-Mesilla Bolson: 9,000 feet (Hueco), 2,000 feet (Mesilla)
  • Cenozoic Pecos Alluvium: 1,500 feet

2.2 kilometers is about 7,200 feet, so most of these aquifers wouldn't be deep enough. Even the lower depths of some of the large ones contain dissolved solid minerals, that could be hazardous to human health. The USGS National Water-Quality Assessment (NAWQA) Program has a variety of maps and data detailing water quality nationwide. Various sources of terror for groundwater include

  • Nitrate
  • Chloride
  • Miscellaneous "dissolved solids"
  • Mercury
  • Phosphorous
  • Atrazine
  • More miscellaneous pesticides

The water quality can vary by region. For example, this map of nitrate concentration in the High Plains (from (Gurdak et al. (2009)) varies widely, do to agricultural use and other sources. Location is everything when it comes to water quality.

A nationwide map of aquifers can be found here.

There's one last thing we have to address: How will these people access the groundwater? I would wager that they have decent drilling equipment, if they're that far down, so digging a well system (vertical or horizontal) shouldn't be too hard.

  • $\begingroup$ This is very useful! Thanks for a great and well thought out answer! $\endgroup$ – Quiquȅ Nov 24 '15 at 22:45
  • $\begingroup$ @Hanko-Tanks No problem. It made for some interesting reading and research. $\endgroup$ – HDE 226868 Nov 24 '15 at 22:46

Some glacial meltwater can be found trapped in aquifers that are now continental shelves under the ocean, which while not as convenient could be accessed through horizontal drilling. You could also drain many aquifers by drilling "up" to the water table from your underground lair, although the details of either plan depend a lot on where the colony actually is.

If they are ambitious enough and have the technical know how to do so, they may consider the theory that a vast amount of water is thought to be trapped in the Earth's mantle. This water was in the initial clouds of debris that formed the Earth, and is renewed by oceanic water being subducted into the mantle as part of plate tectonics. Some theories suggest that there could be a much as 3X the water in the Earth's oceans down there, trapped as molecules inside mineral crystals.

"How" they could access water trapped in the mantle is a question for the reader....


There is lots of water available underground. As Thucydides pointed out, most water tables are 10-100 ft below ground. Perhaps a bigger question is, how do you keep the water out of your structure? At 7000 ft of depth there will be thousands of psi pressure on any groundwater or petroleum, and it will find cracks and be pushed toward the void of an underground base. The tunnel into the structure will penetrate lots of strata that are water-bearing, and those would need to well sealed and/or continuously pump water out.

Geology matters in all of this too. Coal mines are in sedimentary geology, and have lots of water bearing layers around them, so they must be actively pumped. If you're in igneous rock, like a gold or diamond mine, you'll have less, but still substantial seepage.


As an addition to all the other answers: they don't need new water. Yes it would be much more convenient, but the ISS is currently only importing abut 5% (no source because vague memory) of its water need, the rest is recycled.

Given that, we can easily assume that they can survive on very little extra water for a long long time, meaning that a very very negligible income from some reservoir could suffice for thousands of people.


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