Open the scenario\Text\Civilopedia.txt 

Move down the page (about a quarter of the page) to the section:

;  Units__________________________________________________________________________________________________________ Units

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At the bottom of this section add unit details:

#PRTO_Nature
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^[Nature] is the great leveller of humanity and as such, should not be attacked.
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#DESC_PRTO_Nature
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^Despite much progress by Man to harness the destructive forces of Nature, things have changed little over many millenniums. Nature still has a surprising upper hand.    
