Thawing Glaciers does not have summoning sickness.
Archelos is untapped.
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 untapped due to Archelos.
The land enters the battlefield, triggering Retreat to Coralhelm, untapping Thawing Glaciers.
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.