New Zealand’s air force carried out a nearly 20-hour medical evacuation to rescue three Americans from Antarctica, calling it a high-risk mission conducted in freezing temperatures and total darkness.

The U.S. National Science Foundation requested the evacuation after three staff members at McMurdo Station were unable to receive adequate medical treatment on-site.

One of the individuals required urgent care, while the other two also needed medical attention and were transported as a precaution, according to New Zealand’s air force.

Air Commodore Andy Scott, head of New Zealand’s air component, said mid-winter flights to Antarctica are among the most difficult missions due to shifting weather, extreme cold and ice landings in total darkness.

New Zealand’s air force executed a nearly 20-hour mission this week to evacuate three U.S. researchers in Antarctica. (Royal New Zealand Air Force)

The round-trip mission lasted 19.5 hours, with the aircraft touching down in Christchurch on Wednesday morning.

Temperatures at McMurdo Station dropped to -11°F during the operation, adding to the already dangerous conditions.

Melissa Sweeney, the U.S. Chargé d’Affaires to New Zealand, praised the operation as “flawless.”

“We are so very grateful. Our Kiwi partners didn’t hesitate to undertake this mission in one of the most unforgiving environments on Earth. Their skill and readiness are truly world-class,” she said.

Reuters contributed to this report.