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 untapped as a copy of Petrified Field.
When Echoing Deeps enters the battlefield, Thranduil, Sindarin Liege triggers.
Holding priority, 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 clever loop transforms a simple land drop into an endless cycle of recursion, anchored by Kodama of the East Tree's ability to cheat permanents into play whenever another permanent enters your board. Thranduil, Sindarin Liege provides the crucial bridge by spitting out an Elf token on every single landfall trigger, instantly triggering Kodama anew. To keep the engine fed, Petrified Field and Echoing Deeps endlessly trade places between the battlefield and your hand by sacrificing themselves to pull each other back out of the graveyard. While Archelos, Lagoon Mystic sits idly on the sidelines in this specific execution, the pairing of the two land-recycling stalwarts with token generation lets you flood the board with infinitely repeating entry effects.