Deserted Temple

Untapping a land for one generic mana sounds like a parlor trick until you point it at something that does more than tap for mana. The colorless mana it produces is incidental; the second ability is the engine, and it scales with whatever you connect it to. Aim it at a high-output source and you have profit; aim it at a land with a powerful activated ability and you have rebought that ability the same turn. This is enabler-grade design, infrastructure that asks the surrounding lands to supply the payoff. The catch keeps it honest: the Temple taps as part of the untap cost, so without another way to untap the Temple itself it fires exactly once per turn. That ceiling is what separates it from a true combo engine; on its own it doubles a land's output for a turn rather than spinning it into a loop, and the loop only materializes when a third piece untaps the Temple. The fixing tax is just as plain: it makes colorless mana by itself, a utility land that sits idle until the rest of the manabase grows worth untapping (anything in the Gaea's Cradle and Cabal Coffers family, for a start). That trade has kept it a deliberate inclusion rather than an auto-include, a card you run because you have built a reason to untap something, not because untapping is generically good.