When Petrified Field enters the battlefield, Kodama of the East Tree and Thranduil, Sindarin Liege trigger.
Resolve the Kodama of the East Tree trigger, putting nothing onto the battlefield.
Holding priority, activate Petrified Field's second ability by tapping and sacrificing it, returning Echoing Deeps from your graveyard to your hand.
Resolve the Thranduil, Sindarin Liege trigger, creating a token.
When the token enters the battlefield, Kodama of the East Tree triggers, putting Echoing Deeps from your hand onto the battlefield as a copy of Petrified Field.
When Echoing Deeps enters the battlefield, Thranduil, Sindarin Liege and Stone-Seeder Hierophant trigger.
Resolve the Stone-Seeder Hierophant trigger, untapping it.
Activate Echoing Deeps using Petrified Field's second ability by tapping and sacrificing it, returning Petrified Field from your graveyard to your hand.
Resolve the Thranduil, Sindarin Liege trigger, creating a token.
When the token enters the battlefield, Kodama of the East Tree triggers, putting Petrified Field from your hand onto the battlefield.
When Petrified Field enters the battlefield, Thranduil, Sindarin Liege triggers.
Repeat from step 4.
This elegant loop turns a simple land drop into an engine of endless growth by ping-ponging the same two land cards back and forth from the grave. Kodama of the East Tree serves as the mechanical engine, using the token generated by Thranduil, Sindarin Liege's landfall ability to continuously smuggle lands into play from your hand. Meanwhile, Stone-Seeder Hierophant provides the crucial untap utility needed to keep the cycle going. By having Petrified Field and Echoing Deeps repeatedly sacrifice themselves to pull each other out of the graveyard, you generate an infinite chain of tokens, landfall triggers, and creature entries.