You have a basic Forest, Mountain, or Swamp in library.
Steps
Play
Riveteers Overlook.
Riveteers Overlook enters the battlefield, triggering itself and
Retreat to Coralhelm.
Resolve the
Riveteers Overlook trigger, causing you to sacrifice it, then search your library for a basic Mountain, Swamp or Forest card, put it onto the battlefield tapped, and gain 1 life.
The basic land enters the battlefield, triggering
Retreat to Coralhelm, tapping
Scaretiller.
Scaretiller triggers, returning
Riveteers Overlook from your graveyard to the battlefield tapped.
Riveteers Overlook enters the battlefield, triggering itself and
Retreat to Coralhelm.
Resolve the
Retreat to Coralhelm trigger, untapping
Scaretiller.
Repeat from step 3.
Results
Near-infinite landfall triggers.
Near-infinite lifegain.
Near-infinite lifegain triggers.
Put all basic Swamps, Forests, and Mountains from your library onto the battlefield tapped.