AMUNDSEN-SCOTT SOUTH POLE STATION, Antarctica (Reuters) - The 300 Club is not for the faint of heart. Or the cold of blood. This traditional winter rite at the South Pole is considered slightly unusual even by those who have earned the right to call themselves members of the club.

The Trans Antarctic Mountains are clearly visible from the flight deck of a ski-equipped cargo plane flying from McMurdo Station to Amundsen-Scott South Pole Station in Antarctica, December 11, 2006. REUTERS/Deborah Zabarenko

One of those is Andy Martinez, the South Pole winter site manager.

Membership requirements are simple and brutal: The temperature outside at the South Pole must be -100 degrees F (-73 degrees C) or lower -- without factoring in wind chill, Martinez said.

To qualify, initiates must sit in a sauna cranked up to 200 degrees F (93 degrees C) for as long as they can stand, then run -- naked except for a pair of insulated boots and an optional neck gaiter -- around the South Pole and back. That means club members experience a 300-degree swing in temperature.

“You run around the pole, hitting all 24 time zones, and you go back in your birthday suit,” he said.

Underwear is discouraged, Martinez said, because it tends to get damp with perspiration in the sauna and then instantly freezes when the wearer hits the sub-sub-zero air. Cuts from boxer shorts can be a hazard.

The neck gaiter goes over nose and mouth to prevent the cold air from freezing the lungs, he said.

It was 104 degrees below zero F (-75.5 degrees C) the year Martinez joined the club. The wind chill made it feel like -144 degrees F (-97.7 degrees C).

“It just felt like somebody was hitting me with a tennis racket full of needles,” he recalled.

Will he try it again this coming winter?

“I say no, but -- peer pressure.”