Basalt Monolith + Katsumasa, the Animator + Training Grounds

Basalt Monolith

Katsumasa, the Animator

Training Grounds

How it works

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

By turning Basalt Monolith into an artifact creature with Katsumasa, the Animator, we open the door for Training Grounds to drastically discount its untap ability. Because Training Grounds reduces that activation cost from three mana down to just one, tapping the Monolith for three colorless mana leaves you with a net profit every single cycle. This lets you endlessly loop the Monolith's tap and untap abilities to generate infinite colorless mana.