Activate Walking Atlas by tapping it, putting a land card from your hand onto the battlefield.
When the land enters the battlefield, Retreat to Coralhelm and Omnath's second ability triggers.
Resolve the Retreat to Coralhelm trigger, untapping Walking Atlas.
Resolve the Omnath trigger, putting a +1/+1 counter on an Elemental you control, and then causing you to reveal cards from the top of your library until you reveal a land card and put it into your hand due to Abundance.
Repeat.
Results
Near-infinite landfall triggers.
Put all lands from your hand and library onto the battlefield.