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.