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Broadly speaking you need three things: (1) something that makes it infeasible to keep living on land (most people won't move unless pushed), (2) the inability to evacuate from the planet, and (3) ...
Answer
Broadly speaking you need three things: (1) something that makes it infeasible to keep living on land (most people won't move unless pushed), (2) the inability to evacuate from the planet, and (3) ...