Voltaic Construct + Incubation Druid + Transmogrifying Licid

Voltaic Construct

Incubation Druid

Transmogrifying Licid

How it works

1. Activate Incubation Druid by tapping it, adding three mana of any type a land you control can produce
2. Activate Voltaic Construct by paying 2, untapping Incubation Druid.
3. Repeat

To pull off this infinite mana engine, Incubation Druid needs to be both an artifact and fueled by enough mana to sustain its own untaps. That is where Transmogrifying Licid works its behind-the-scenes magic, turning the Druid into an artifact creature so it can be targeted by Voltaic Construct's untap ability. Once the Druid has its +1/+1 counters to tap for three mana, you simply use two of that mana to pay for Voltaic Construct, pocketing a profit of one mana each cycle.