Earthcraft

A two-mana enchantment that quietly broke the rule that mana is a finite resource. The trade it offers looks fair on paper: tap a creature, untap a land. But the moment that land produces mana and you have a second way to untap the creature, the loop closes and the engine spins for free. The history of this card is the history of the people who closed it. Squirrel Nest is the canonical partner: every Squirrel token it makes is a creature to tap, and each token costs only an Earthcraft activation, so a single basic Forest builds an unbounded army of creatures. (The mana itself is the prize when the loop is fed through a creature that taps for mana instead of a token engine; on the Squirrel Nest line specifically, the Forest is spent making bodies, not piling up extra mana.) That interaction got Earthcraft banned in Legacy, and it remains restricted in Vintage, a rare distinction for a green enchantment policed alongside the format's fast mana and broken artifacts.

What makes the design so volatile is the asymmetry between cost and ceiling. The activation has no mana cost, no tap symbol on the enchantment itself, and no cap on how many times it fires per turn; the only friction is needing untapped creatures and the basic-land restriction. Designers have spent the decades since printing creature-untaps and land-untaps with deliberate guardrails (summoning-sick clauses, mana costs, once-per-turn limits) precisely because this card showed what happens when you ship one without them.