Basalt Monolith + Biomancer's Familiar + Karn, the Great Creator

Basalt Monolith

Biomancer's Familiar

Karn, the Great Creator

How it works

1. Activate Karn's first loyalty ability by putting one loyalty counter on them, making Basalt Monolith a 3/3 artifact creature until your next turn.
2. Activate Basalt Monolith's first ability by tapping it, adding CCC.
3. Activate Basalt Monolith's last ability by paying 2, untapping it.
4. Repeat from step 2.

By using Karn, the Great Creator to animate Basalt Monolith into an artifact creature, it gains a creature type that unlocks the cost-reduction power of Biomancer's Familiar. Normally, it costs three mana to untap Basalt Monolith, but the Familiar reduces that activation cost to just one mana. Because tapping the Monolith generates three colorless mana while untapping it only costs one, you are left with a net gain of two mana each cycle. This loop can be repeated indefinitely to generate infinite colorless mana.