Hullbreaker Horror + Mana Crypt

Steps

1. Activate Mana Crypt by tapping it, adding {C}{C}.
2. Cast the permanent that can be cast using {C}{C} from your hand by paying its mana cost.
3. Hullbreaker Horror triggers, returning Mana Crypt from the battlefield to your hand.
4. Cast Mana Crypt by paying {0}.
5. Hullbreaker Horror triggers, returning the permanent cast in step 2 from the battlefield to your hand.
6. Repeat.
7. If the permanent can be cast using {C} or less, this combo nets infinite colorless mana.