Voltaic Construct + Viridian Joiner + Neurok Transmuter

Voltaic Construct

Viridian Joiner

Neurok Transmuter

How it works

1. Cause Viridian Joiner to become an artifact using Neurok Transmuter.
2. Activate Viridian Joiner by tapping it, adding at least 3.
3. Activate Voltaic Construct by paying 2, untapping Viridian Joiner.
4. Repeat from step 2.

This clever loop relies on turning Viridian Joiner into an artifact creature with Neurok Transmuter so that it can be targeted by Voltaic Construct's untap ability. Once the joiner's power is high enough to produce more mana than the construct requires to untap it, tapping it for green mana and paying to untap it creates a self-sustaining engine. Because the Transmuter's effect only lasts until end of turn, you simply need to keep it artifact-ized for the duration, yielding as much green mana and as many artifact creature untaps as you desire.