From the Super Mario Wiki
The Shiver Snowfield are three paths in the Shiver Region linking together Shiver City, Shiver Mountain, and Starborn Valley. In Paper Mario, Mario needs to travel through here to eventually get to the Crystal Palace. A long line of snowmen in the center path guards the secret door into Shiver Mountain, the items needed to open the door are in the city and valley. Just before Starborn Valley, solar radiation resembling the Aurora Borealis (Northern Lights) can be seen in the sky.
The in-game map description reads, "This icy snowfield links Shiver City with Starborn Valley. A path leads from the snowfield to Shiver Mountain. By going through the mountain, you can reach Crystal Palace."
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