Mishra's Groundbreaker

Four mana buys an artifact whose only job is to sit, untap, and convert itself into a permanent threat: tap it, sacrifice it, and one of your lands becomes a 3/3 artifact creature for the rest of the game while still tapping for mana between attacks. Because the artifact has no summoning sickness of its own, you can deploy and fire it on the same turn, but you are still trading the whole card plus an activation for a body that lives on top of your mana base. That is the design tension. Animating a land with an indefinite effect, so the land stays a land, was a genuinely novel trick for the era, and the conversion rate is the honest cost: you fold a creature into a mana source and accept that the two now share a single point of failure. The animated land is exposed to every piece of creature removal on the table, and answering it costs you a land rather than just a body; a board wipe takes both at once. This is slow, deliberate land-animation design from a period when Wizards was still mapping the boundaries between artifact, land, and creature, before manlands settled into a clean template that put the body and the mana on the same card without the up-front tax or the sacrifice.