Easily one of the rarest medals a troop can earn is the Antarctica Service Medal. Spend a single day in Antarctica, south of 60 degrees latitude in the Southern hemisphere, and you've forever got bragging rights.
Do it in the wintertime and you've earned a distinctive "Wintered Over" clasp you can hold over everyone else. But being authorized to get down there is the hardest part.
Since its discovery in 1820, numerous nations who've landed in Antarctica have stuck their flags in the ground and claimed it as their own. Because the continent has essentially no readily available resources, is extremely remote, and was nearly impossible to settle on long-term, the flags (and their claims) were fairly weak.
But that didn't stop many nations from trying to hold a claim. The United Kingdom (and, by extension, Australia and New Zealand), France, Norway, Argentina, Chile, and Nazi Germany all claimed portions of the continent. The United States and the USSR also held the right to make a claim but never did.
To ease tensions between all parties in 1959, the Antarctica Treaty was established which laid the ground rules for the continent. It was agreed that Antarctica is the "common heritage of mankind" and could not belong to an entity, territorial claim or not.
This was established to increase scientific understanding of the region and allow scientists the ability to freely communicate. Another article of the treaty bans military personnel and nuclear weapons testing from the continent.
The only exception to this policy is that troops are allowed entry into Antarctica as long as it's done for scientific research and other peaceful purposes — this is the exact mission of every troop who travels south of the 60-degree line.
Airmen, sailors, and coast guardsmen will routinely travel to scientific research facilities to give aid, transportation, or supplies. However, finding the justification to send soldiers or Marines is more limited.
These troops can be found at McMurdo Station, one of the largest coastal facilities on the continent, and Amundsen-Scott South Pole Station, which is a scientific research facility located at the geographic south pole.