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.