Retreat to Coralhelm + Dimir Aqueduct + PuPu UFO

Retreat to Coralhelm

Dimir Aqueduct

PuPu UFO

How it works

1. Activate PuPu UFO by tapping it, putting Dimir Aqueduct from your hand onto the battlefield.
2. When Dimir Aqueduct enters the battlefield, it and Retreat to Coralhelm trigger.
3. Resolve the Dimir Aqueduct trigger, returning itself from the battlefield to your hand.
4. Resolve the Retreat to Coralhelm trigger, untapping PuPu UFO.
5. Repeat.