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Keeping it on mars, one easy reason is flooding. If they are there teraforming, things start to warm up, a lot of permafrost will melt. That water has to go somewhere, and the most likely place is ...
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#1: Attribution notice removed
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System
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2021-03-30T23:53:18Z (over 3 years ago)
Source: https://worldbuilding.stackexchange.com/a/69669 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/