Animate a Forest that enters untapped using Druid Class
Cast Springheart Nantuko for its bestow cost by paying 1, attaching it to the animated Forest.
Play a land. When the land enters, Springheart Nantuko triggers, causing you to pay 1 to create two token copies of the Forest that enters untapped.
When the Forest tokens enter, Springheart Nantuko triggers twice.
Holding priority, activate the Forest tokens by tapping them, adding .
Resolve the first Springheart Nantuko trigger, choosing not to pay and creating two 1/1 Insect creature tokens.
Resolve the second Springheart Nantuko trigger, causing you to pay 1 to create two token copies of the Forest that enters untapped.
Repeat from step 4.
By turning a Forest into an animated creature via Druid Class, you set the stage for Springheart Nantuko to bestow its power upon it, allowing every land drop to trigger a copy of your land-creature. This engine goes exponential thanks to Primal Vigor, which doubles every token generated by the Nantuko's landfall ability—including the copies of your animated land, which instantly enter untapped and can be tapped for mana to sustain the loop indefinitely.