How do units get Altitude Training in Civilization 5?

How do units get Altitude Training in Civilization 5? - Unrecognizable mountaineer climbing on cliff

Sometimes my units in Civilization 5 get "altitude training" promotion which makes movement faster. However, I have no idea where it comes from. Is it randomly assigned to some units or is there some way to improve the chances of getting that promotion?

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★ Civilization 5 - Deity #8 - The Secret and Advanced Strategies!

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