Sun Titan + Helvault + Krark-Clan Ironworks = infinite ETBs and colorless mana
How it works:
- Have Sun Titan, Helvault, and Krark-Clan Ironworks on the battlefield
- Activate Helvault's first ability, exiling Sun Titan (mana: -1)
- Sacrifice Helvault to Krark-Clan Ironworks, making 2 colorless mana and returning Sun Titan to the battlefield (mana: +1)
- Use Sun Titan's ETB ability to return Helvault to the battlefield
- Repeat