Veldt

The depletion lands of Ice Age read cleaner than they play: a dual that fixes either of two colors but charges a tempo tax for the privilege, and the counter is the meter. Tap the land and it picks up a depletion counter, which keeps it from untapping. The clear comes at upkeep, but the untap step runs first, so the counter does not vanish in time to free the land that turn: it sits tapped for the entire next turn and only untaps the turn after. The effective rate is one colored mana every other turn from a source that produces nothing on its off cycle. That cadence is the experiment Wizards was running here: lands that fixed colors generously while taxing the rhythm of your turns, a tradeoff later duals (the Apocalypse painlands, the Ravnica shocklands, the various taplands) would solve in steadier, less swingy ways. The depletion mechanic never returned in this form, which tells you how the trial landed. The awkwardness is that the off-turn is not free downtime: a land you cannot tap cannot hold up a response or feed a turn's worth of plays, and a manabase leaning on these stutters in a way modern fixing does not. Veldt is the green-white member of the group, for decks that wanted both colors badly enough to eat the stutter, from a moment before "enters tapped" had settled in as the standard lever for pricing a dual.