Ancient Den

Two words in a type line do all the strategic work here: this produces white mana like a Plains, but the word "Artifact" sitting above the line is what made it indispensable to the affinity-and-metalcraft engine of its era. Each copy was a free pip toward Frogmite, Myr Enforcer, and the affinity discount while still fixing white, a land that quietly held down two jobs at once. The cost of that second identity is that it answers to any artifact-targeted Stone Rain effect, a liability a Plains never carries.

For years it was a four-of in any white-touching affinity build, and that ubiquity is exactly what got it pulled. The artifact lands were banned in Standard when affinity proved too fast and too consistent, and they have lived under restriction in eternal formats ever since: legal in some, banned where artifact-count and recursion proved unsafe. The design tension never resolved cleanly. Wizards has stayed cautious about returning the full cycle, which is why a permanent whose entire printed text is a mana ability still carries a ban-list footnote. The danger was never in the ability line; it was in the type line above it, where a single word doubles a land's value and its exposure in the same breath.