Urza, Lord High Artificer, Metalworker, and Umbral Mantle

Prerequisites

• All permanents on the battlefield
• Metalworker does not have summoning sickness
• Umbral Mantle attached to Metalworker
• You have at least two additional artifacts in hand

Steps

1. Tap Metalworker to reveal 2 artifact cards from your hand add a total of 4
2. Spend to untap Metalworker with Umbral Mantle's ability and give it +2/+2 until the end of the turn
3. Repeat for infinite colorless mana
4. Repeat those steps tapping Metalworker through Urza's ability for for infinite blue mana
5. Spend some of said infinite mana to exile your library to Urza's ability, enabling you to play all of those cards until the end of your turn

Results

• Infinite colorless mana
• Infinite blue mana
• Cast any cards from your library with blue or colorless casting costs