Activate Metalworker by tapping it and revealing at least three artifact cards from your hand, adding at least six ({C} magic symbol) .
Activate Urza by paying ({6} magic symbol) , creating a token copy of Seat of the Synod that is a 1/1 Soldier.
When the Seat of the Synod token enters the battlefield, Retreat to Coralhelm triggers, untapping Metalworker.
Repeat.
If you have four or more artifact cards in hand, this combo nets infinite colorless mana, and you may target any artifact you control to get infinite copies of them.