The Mine Shack.
The Mine Shack is a secret area that can only be accessed from the Map or by finishing a game of Cart Surfer. It is simply an entrance to the Mine.
History
No one knows who built the Mine Shack. It was discovered in the summer of 2006 after the opening of the Underground.
The story goes that explorers hopped on a mine cart and followed the track to see where it would lead. After a zigzagging ride, they ended up outside in a pile of snow near the mountains of Club Penguin. They looked back at their exit and thought it looked like a doorframe. It was covered in a lot of snow. By the time they had finished digging, they had uncovered the Mine Shack. No one knows how long it was there, covered in piles of snow.