If I am playing a deck with Snow-Covered Mountains, does a creature with Mountainwalk become unblockable or do they require Snow-Covered Mountainwalk (where "mountain" is interchangeable with plains, islands, swamps, forests and mountains)? Conversely, if I had a creature with Snow-Covered Mountainwalk, is it unblockable if an opponent has only regular Mountains in play?
I lack the experience with the Comprehensive Rules to be able to conclusively go one way or the other.
702.12a Landwalk is a generic term that appears within an object’s rules text as “[type]walk,” where [type] is usually a subtype (as in “islandwalk”), but can be the card type land, any land type, any supertype, or any combination thereof (as in “snow swampwalk” or “legendary landwalk”).
702.12b Landwalk is an evasion ability. A creature with landwalk is unblockable as long as the defending player controls at least one land with the specified subtype and/or supertype. (See rule 509, “Declare Blockers Step.”)
I suppose the question boils down to this: is "Mountain" a supertype of "Snow-Covered Mountain"?