Karn, the Great Creator, Voltaic Construct, and Grim Monolith

Prerequisites

• Karn, the Great Creator on the battlefield
• Voltaic Construct on the battlefield
• Grim Monolith on the battlefield
• Grim Monolith does not have summoning sickness

Steps

1. Activate Karn's first loyalty ability, causing Grim Monolith to become an artifact creature until end of turn
2. Activate Grim Monolith by tapping it, adding
3. Activate Voltaic Construct by paying , untapping Grim Monolith
4. Repeat from step 2

Results

• Infinite colorless mana