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From MTG Wiki
One of the five basic lands, Mountain provides red mana, which is required to cast most red spells. 
Additionally there is a snow variant, the Snow-Covered Mountain.
Mountain has been printed in more sets than any other card, including the other basic lands. It was the only basic land printed in Arabian Nights. An Arabian Nights mountain is the rarest mountain from any set. 
The island of Shiv is the most powerful red mana location on Dominaria.