Goblin Welder + Retreat to Coralhelm + Darksteel Citadel + Ancient Den

Goblin Welder
Retreat to Coralhelm
Darksteel Citadel
Ancient Den

Requires
Darksteel Citadel in your graveyard. All other permanents on the battlefield. Goblin Welder does not have summoning sickness.
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 c
  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 w
  7. Repeat
Results
Infinite white mana. Infinite colorless mana. Infinite landfall triggers.