Snow-Covered Mountain

The basic land with a data type all its own. Functionally it taps for red like any Mountain, but the snow supertype is a hidden parameter that snow-matters cards read off of: it is the same red mana, carrying an invisible flag. That flag is the entire reason this exists. It lets a deck pay the cost of snow synergies (Skred scaling off snow permanents, snow filtering, mana abilities that count snow sources) without conceding anything on the manabase, because a Snow-Covered Mountain is strictly a Mountain plus a label. The design tension Wizards manages every time it reprints the snow basics is exactly that "free upside" problem: a land that does its normal job and also quietly fuels a payoff is a deckbuilding decision with no opportunity cost, so snow payoffs have to be priced as if every red source could be snow. The supertype also changes a subtle rules edge: effects that fetch or care about basic Mountains still see it (it is a basic Mountain), while effects that count snow permanents see something a printed Mountain does not. It is one of the cleanest examples in the game of a card whose entire identity lives in a single word that does nothing until another card asks about it.