Toph, the First Metalbender + Caged Sun

Prerequisites

You control a mana-producing land who can produce mana of the color named by Caged Sun.

Steps

1. Activate a land by tapping it, adding at least one mana of the color named by Caged Sun.
2. Caged Sun triggers, adding one mana of that color.
3. Caged Sun triggers again, as it is a land due to Toph, adding one mana of that color.
4. Step 3 repeats in a mandatory infinite loop, causing the game to end in a draw.