Rex, Cyber-Hound, Trench Behemoth, and Walking Atlas

Prerequisites

• Rex, Cyber-Hound on the battlefield
• Trench Behemoth in exile
• Walking Atlas in exile

Steps

1. Activate Rex using Walking Atlas's ability by tapping it, putting a land from your hand onto the battlefield
2. If the land is mana-producing and enters the battlefield untapped, you may activate it by tapping it, adding at least
3. Activate Rex using Trench Behemoth's ability by returning the land from the battlefield to your hand, untapping Rex
4. Repeat

Results

• Infinite landfall triggers
• Infinite mana lands you control that enter the battlefield untapped can produce