The United States Antarctic Program operates 3 permanently manned bases in the Antarctic: Palmer Station, Amundsen-Scott South Pole Station and McMurdo Station. Palmer is the smallest permanent American base and resides within the Chilean/Argentinian/British claim and Amundsen-Scott is the only Antarctic base located at the South Pole. McMurdo is a huge year round base in the Ross Dependency with a summer population of just over 1200. It is the largest base in the Antarctic.
Philatelic Mail Clerk McMurdo Station, Antarctica PSC 469 APO AP 96599-1035 UNITED STATES Philatelic Mail Clerk Amundsen-Scott South Pole Station, Antarctica PSC 468 Box 400 APO AP 96598 UNITED STATES Philatelic Mail Clerk Palmer Station, Antarctica c/o AGUNSA Deposito Franco Antartico P.O. Box 60-D Punta Arenas CHILE
Note that McMurdo and Amundsen-Scott are APO (Army Post Office) addresses and you can treat them as being addresses in the United States. However Palmer Station mail is routed through Chile so when sending your mail there be sure to use the correct international postage rate.
The last cover I sent to Palmer came back canceled in Denver, Colorado - home to Raytheon Polar Services Company, who were the NSF operations support contractor at the time. So perhaps mail from Palmer doesn't have a direct path into the international mail stream. They likely do not have an acting postmaster or post office. Or, maybe if I had put Chilean postage on my cover and asked nicely, it would have been sent back to me through Correos Chile. I'll have to try that someday.
Additional information about polar philately and other stamp-related topics can be found in my Blog
last updated 30 March 2014