Buzz Aldrin says he was evacuated from the South Pole last week because he became short of breath and began showing signs of altitude sickness.
The 86-year-old adventurer, who was the second man to walk on the moon, released details on Sunday of his dramatic medical evacuation from Antarctica. He is continuing to recuperate in a hospital in Christchurch, New Zealand.
Because of the thick ice that blankets Antarctica, the South Pole sits at an elevation of 2835 metres.
Aldrin said in a statement on Sunday he still has some congestion in his lungs and so has been advised to rest in New Zealand until it clears up and to avoid the long flight back to the US for now.
Aldrin, his son Andrew and manager Christina Korp had been visiting Antarctica as tourists on a trip organised by the White Desert tour company. They left last Tuesday from South Africa.
"South Pole here I come!" Aldrin wrote on Twitter.
He said the trip began well, and that he'd been planning to spend time with scientists who were studying what it would be like to live on Mars because the conditions in Antarctica were similar.
"I had been having a great time with the group at White Desert's camp before we ventured further south," he said.
"I started to feel a bit short of breath so the staff decided to check my vitals. After some examination they noticed congestion in my lungs and that my oxygen levels were low, which indicated symptoms of altitude sickness."
Aldrin said he was put on the next flight, a ski-equipped LC-130 cargo plane that took him to McMurdo Station, a US research centre on the Antarctic coast.
"Once I was at sea level I began to feel much better," he said. From there he was flown to Christchurch, where he arrived at 4.30am on Friday. His manager described the evacuation as "gruelling". She posted a picture of Aldrin on his hospital bed, on oxygen and with an IV in his left arm.
The following day he was looking upbeat as he welcomed NASA Deputy Administrator Dava Newman.
Aldrin posted a photo of himself wearing pyjamas and sitting up in bed with Newman by his side. Both were giving the thumbs-up sign.
"I had a surprise visitor this morning. My longtime friend," Aldrin wrote. "She beat me to the South Pole by one day."
Korp, his manager, wrote on Twitter that "Buzz is ok. Resting but flirting with all the nurses!"
She said she'd told Aldrin he now holds the record as the oldest person to reach the South Pole, according to the National Science Foundation.
"He'll be insufferable now," she wrote.
Indeed, the achievement completes a remarkable list of extreme locations visited by Aldrin. In 1966, he spacewalked while in orbit. In 1969, he and Neil Armstrong became the first men to walk on the moon.
In 1996, Aldrin voyaged to the bottom of the North Atlantic in a mini-submarine to view the Titanic.
A couple of years later, he travelled to the Arctic aboard a Russian icebreaker.
Aldrin is now focusing on getting back to his home in Satellite Beach, Florida, which is not far from Cape Canaveral and its launch pads. But he's eyeing more distant horizons.
"I'm looking forward to getting home soon to spend Christmas with my family and to continue my quest for a permanent settlement on Mars," he said in his statement.
"You ain't seen nothing yet!"