The Southern Tundra is the southern-most continent of the planet Mobius, constantly featuring to frozen temperatures and snow storms.
A largely barren, mysterious region, the Southern Tundra first saw visitors during the Forgotten War between the Albion Knights of Aurora and the Order of Ixis. When the conflict ended, few traces remained of the feuding groups, though some structures apparently survived. (CSE)
For a while it acted as the home to the self-exiled Overlander Nate Morgan and his friend Eddy the Yeti, though after his castle-a remnant of the Order of Ixis' presence-was destroyed in a battle between Sonic, Tails and Ixis Naugus resulting in Eddy's death, Nate returned with the two heroes to Knothole. (StH: #64, #65, #66, CSE)
Sonic briefly revisited this area again while trying to undo the effects of a time-altering beam with Nate's Super Emerald strapped to his back. At that point, he was in the form of Polar Sonic. (StH: #71)
Some time later, Sonic returned to the southern tundra to place a rose on Eddy's grave, only to be attacked by the recently escaped Naugus once again. Sonic defeated Naugus however, once again imprisoning him in the Zone of Silence before returning to Knothole. (SSS: #15 and FCBD: #5 )
Most recently, the Xorda dispatched the Quantum Dial in an attempt to destroy the Mobian solar system. Nearly every living Mobian (and one deceased Mobian) rushed to the southern tundra to stop the dial. The battle ended when Sonic used his speed to implode the dial, warping him to the planet Thoraxia, causing everyone to believe he had died. (StH: #125)