[ericthered]

One of my favorite disasters:

A form of algae that floats in the sky via tiny gasbags appears, whether evolved or created. its very small, but doesn't seem to need soil and can pull water from the atmosphere. Clouds of this stuff slowly block out the sun, lowering SURFACE temperatures on earth and causing crop failures.

If you show up after its happened, you just get oppressive clouds over a surface as lifeless as the deep sea floor -- which is to say that things survive, but its a different world. Please note this disaster is quite possibly contagious.

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Nuclear wars are a nice way to build a hell world --- radiation everywhere, nuclear winter, and so forth.

Actually, ATE is essentially a book on hell worlds.