Voltaic Construct + Katsumasa, the Animator + Basalt Monolith

Voltaic Construct

Katsumasa, the Animator

Basalt Monolith

How it works

1. Animate Basalt Monolith using Katsumasa.
2. Activate Basalt Monolith by tapping it, adding CCC.
3. Activate Voltaic Construct by paying 2, untapping Basalt Monolith.
4. Repeat from step 2.

To pull off this infinite mana loop, you first need to enlist Katsumasa, the Animator to turn your Basalt Monolith into an artifact creature so it's a valid target for untapping. Once animated, tapping the Basalt Monolith yields three colorless mana, which easily funds Voltaic Construct's ability to untap the Monolith for just two mana. Because Basalt Monolith produces more mana than the Voltaic Construct requires to untap it, you net one colorless mana with each iteration, fueling an engine that generates infinite colorless mana and infinitely untaps your artifact creatures.