Goblin Welder, Retreat to Coralhelm, Darksteel Citadel, and Ancient Den

Prerequisites

• Goblin Welder on the battlefield
• Retreat to Coralhelm on the battlefield
• Darksteel Citadel in the graveyard
• Ancient Den on the battlefield

Steps

1. Activate Goblin Welder by tapping it, causing you to sacrifice Ancient Den and return Darksteel Citadel from your graveyard to the battlefield
2. Darksteel Citadel enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder
3. Activate Darksteel Citadel by tapping it, adding
4. Activate Goblin Welder by tapping it, causing you to sacrifice Darksteel Citadel and return Ancient Den from your graveyard to the battlefield
5. Ancient Den enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder
6. Activate Ancient Den by tapping it, adding
7. Repeat

Results

• Infinite colorless mana
• Infinite landfall triggers
• Infinite white mana