Voltaic Key, Forsaken Monument, Rings of Brighthearth, and Mana Vault

Prerequisites

• Voltaic Key on the battlefield
• Forsaken Monument on the battlefield
• Rings of Brighthearth on the battlefield
• Mana Vault on the battlefield

Steps

1. Activate Mana Vault by tapping it, adding
2. Forsaken Monument triggers, adding
3. Activate Voltaic Key by paying and tapping it, targeting itself
4. Rings of Brighthearth triggers, causing you to pay to copy Voltaic Key's ability, targeting Mana Vault
5. Resolve both Voltaic Key abilities, untapping it and Mana Vault
6. Repeat

Results

• Infinite untap of all artifacts you control