Goblin Welder + Retreat to Coralhelm + Ancient Den + Great Furnace

Goblin Welder
Retreat to Coralhelm
Ancient Den
Great Furnace

Requires
Ancient Den 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 Great Furnace and return Ancient Den from your graveyard to the battlefield
  2. Ancient Den enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder
  3. Activate Ancient Den by tapping it, adding w
  4. Activate Goblin Welder by tapping it, causing you to sacrifice Ancient Den and return Great Furnace from your graveyard to the battlefield
  5. Great Furnace enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder
  6. Activate Great Furnace by tapping it, adding r
  7. Repeat
Results
Infinite white mana. Infinite red mana. Infinite landfall triggers.