A rescue plane has landed at the South Pole to evacuate a sick worker from a remote US science station.
The plane arrived at the Pole after a daring 1,500-mile, nine-hour trip from a British base on the Antarctic peninsula, according to the National Science Foundation, which runs the polar outpost.
The plane's crew - a pilot, co-pilot, flight engineer and medical worker - will rest and wait for at least 10 hours. Then, if weather conditions are favourable, the plane will refuel and return to Rothera, said agency spokesman Peter West.
After that the sick worker will be taken out of Antarctica for treatment.
"It went all according to plan," Mr West said from Arlington, Virginia.
A second worker is also ill, but officials have yet to decide whether that patient will also fly out, Mr West said.
The science foundation will not identify the workers, who are employees of Lockheed Martin, which handles logistics at the station, nor their medical conditions.
There have been three emergency evacuations from the Amundsen-Scott station since 1999. Workers at the South Pole station are isolated from February through to October, the coldest and darkest months when it is too risky for routine flights.
The latest mission is pushing the limits of what is acceptable, said Tim Stockings, operations director at the British Antarctic Survey in London. He said being prepared is key.
"The air and Antarctica are unforgiving environments and punishes any slackness very hard," Mr Stockings said. "If you are complacent it will bite you."
"Things can change very quickly down there", he said, with ice from clouds, high winds and snow.
The first day of winter in the Southern Hemisphere was Monday - the sun will not rise at the South Pole until the first day of spring in September.
The National Science Foundation decided last week to mount the rescue operation because one staffer needed medical care that cannot be provided there.
The station has a doctor, a physician's assistant and is connected to doctors in the US for consultations, Mr West said. There are 48 people - 39 men and nine women - at the station.
The temperature on Tuesday afternoon at the South Pole station was minus 60C, with a wind chill that makes it feel like minus 78C according to the science foundation's weather station.
The extreme cold affects a lot of things on planes, including fuel, which needs to be warmed before takeoff, batteries and hydraulics. The Twin Otter aircraft can fly in temperatures as low as minus 75C.
The 1999 flight, which was done in Antarctic spring with slightly better conditions, rescued the station's doctor, Jerri Nielsen, who had breast cancer and had been treating herself. Rescues were carried out in 2001 and 2003, both for gallbladder problems.
Scientists have had a station at the South Pole since 1956. It carries out research in astronomy, physics and environmental science with telescopes, seismographs and instruments that monitor the atmosphere.
The foundation runs two other science stations in Antarctica.