The Royal New Zealand Air Force evacuated three people needing medical care from a U.S. National Science Foundation base in Antarctica this week.

The evacuees were working at McMurdo Station, operated by the U.S. Antarctic Program under the purview of the National Science Foundation. One person needed urgent care and two others also needed medical help, the New Zealand air force said, though officials didn’t say what conditions prompted Tuesday’s evacuation.

Antarctica is in the middle of winter right now, and program personnel at McMurdo Station had to create a runway out of the ice for the New Zealand plane to land on in the dark overnight, air force officials said.

The plane took just over 19 hours to fly to Antarctica, pick up the passengers and fly back, landing in Christchurch, New Zealand, on Wednesday morning.

The patients, none of them publicly named by authorities, then received treatment in the country’s second-largest city.

“Landing a large aircraft on ice, in darkness, using night vision, takes precision and extraordinary skill. We are deeply grateful for the crew’s professionalism, courage and partnership. This mission was nothing short of heroic,” the U.S. Embassy in New Zealand posted on X.