Basalt Monolith

The original break-even mana rock, and the one that taught Magic what "doesn't untap during your untap step" actually means as a design lever. Tap for three colorless, pay three to untap: the net is zero mana and zero cards, but the loop converts colored mana into colorless and stores production for later, which is enough to anchor the combo archetype it spawned. Pair it with anything that reduces the untap cost or doubles the tap output (Power Artifact and Rings of Brighthearth are the canonical pieces, with Forsaken Monument and Mana Reflection working the production side) and the rock becomes an arbitrarily large colorless engine. That is the design fingerprint Alpha left on the game: a card whose printed rate is deliberately a wash, balanced by the assumption that nothing else in the environment would break the symmetry. Thirty years of design proved that assumption hopelessly optimistic. Every "doesn't untap" artifact since has been costed against the Monolith's lesson, which is that the untap clause is not a drawback in a vacuum; it is a drawback only until someone prints the untapper. The card itself has barely changed: same three-mana cost, same three-mana tap, same three-mana untap, the cleanest symmetry in the cycle.