Lumra, Bellow of the Woods + Mirrorpool + Trenching Steed + Horizon Explorer

Lumra, Bellow of the Woods

Mirrorpool

Trenching Steed

Horizon Explorer

How it works

1. Activate any number of other lands by tapping them, adding at least 4C.
2. Activate Trenching Steed by sacrificing a land except Mirrorpool.
3. Repeat step 2 until you control no lands except Mirrorpool.
4. Activate Mirrorpool's third ability by paying 4C, tapping and sacrificing it, creating a token copy of Lumra.
5. Put the Lumra token into your graveyard due to the legend rule.
6. The Lumra token's enters-the-battlefield ability triggers, causing you to mill four cards and return all land cards from your graveyard to the battlefield untapped due to Horizon Explorer.
7. Repeat.

This crafty loop leverages Mirrorpool to repeatedly clone Lumra, Bellow of the Woods, converting an initial mana investment into a massive cascade of land recursion. By first stripping away your other lands using Trenching Steed, you clear the board so that sacrificing Mirrorpool to copy Lumra immediately triggers the legend rule, sending the copy straight to the graveyard and filling your bin with milled cards. Horizon Explorer makes sure all those returned lands enter untapped, easily refunding the mana needed to keep the cycle going.