Thawing Glaciers does not have summoning sickness.
Steps
Activate Thawing Glaciers by paying ({1} magic symbol) and tapping it, searching your library for a basic land card and putting it onto the battlefield tapped.
The land enters the battlefield, triggering Retreat to Coralhelm and Amulet of Vigor.
Resolve the Retreat to Coralhelm trigger, untapping Thawing Glaciers.
Resolve the Amulet of Vigor trigger, untapping the basic land.
Activate the basic land by tapping it, adding ({1} magic symbol) .
Repeat.
Results
Near-infinite landfall triggers.
Put all basic lands from your library onto the battlefield tapped.