Basalt Monolith, Training Grounds, and Karn, Silver Golem

Prerequisites

• Basalt Monolith on the battlefield
• Training Grounds on the battlefield
• Karn, Silver Golem on the battlefield
• Basalt Monolith does not have summoning sickness

Steps

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

Results

• Infinite colorless mana