Goblin Welder + Retreat to Coralhelm + Seat of the Synod + Ancient Den

Goblin Welder
Retreat to Coralhelm
Seat of the Synod
Ancient Den

Requires
Seat of the Synod 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 Seat of the Synod from your graveyard to the battlefield
  2. Seat of the Synod enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder
  3. Activate Seat of the Synod by tapping it, adding u
  4. Activate Goblin Welder by tapping it, causing you to sacrifice Seat of the Synod 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 blue mana. Infinite landfall triggers.