We have seen in the Matrix trilogy that Zion is a last city of humans, situated at Earth's core where it is still warm.

I believe they found a groundwater source, that's why they have a water purifying plant, but since there is no sign of vegetation above the ground or for that matter in Zion, where did it get its air supply ?

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    This discussion may be of help to you. Nov 25, 2017 at 9:59
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    Interesting discussion but the sci-fi topic explicitly focuses on humans outside of Zion. The asker and respondents all assume that production of oxygen for a small population of humans in a fixed location (such as Zion) would be easily achievable. Nov 25, 2017 at 16:59

2 Answers 2


Machines. While seemingly an inconsequential detail we get a direct quote on this from Councillor Harmann when he discusses the machines keeping Zion alive with Neo:

[Of] course. That's it. You hit it. That's control, isn't it? If we wanted we could smash them to bits. Although, if we did, we'd have to consider what would happen to our lights, our heat, our air...

He doesn't explain the process but since we can already produce oxygenated air for space stations it shouldn't be a huge leap of the imagination.

  • Oxygen can be produced by applying electricity to water (electrolysis). Pretty simple stuff. A lot of hydrogen is produced in the process - twice as much as the oxygen - but that could be ejected to the surface through vents. It could also be used as a source of fuel, but that would burn the oxygen they needed for breathing, so I'd rule that out. Nov 27, 2017 at 19:04

I agree with Sean Condon that the machines produce oxygen. Passing electricity through water splits it into oxygen and hydrogen. Pretty simple stuff. The process is called electrolysis.

So one simple way Zion could be set up is to use an underground river to provide both fresh drinking water, and a source of hydroelectric power. The electricity could be used to split some of the water into hydrogen and oxygen. The hydrogen could be ejected to the Earth's surface, along with excess carbon dioxide.

I suppose hydrogen could be burned as a fuel. But that depletes oxygen, which is needed for breathing, so I'd vote against that. They already have their hydroelectric power source.

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