As the United States enters its coldest season, Antarctica's summer gets into full swing. By November, the sun shines on the ice-covered continent day and night. When it sets in mid-February, Antarctica enters winter, where it's dark for six months. Because of this, summer is Antarctica's busiest period, with thousands of researchers studying this last pristine place on Earth.

Thanks to the Antarctic Treaty, created in 1961, more than 50 countries have agreed to preserve Antarctica for scientific pursuits, exploration and other peaceful purposes. No one owns it, and no one can live there permanently. Scientists who visit cover many disciplines. A biologist might be observing penguins the same day an astronomer looks through the South Pole Telescope. A volcanologist might be exploring an active volcano while a paleoclimatologist drills into ancient ice cores. There's even a residency for artists and writers who are inspired by Antarctica.

But even in summer, Antarctica is a dangerous place. It's the coldest environment on Earth, with a mean temperature of minus-76 in winter and minus-18 in summer, according to the Woods Hole Oceanographic Institution. The surface has so much ice that entire mountains are buried underneath it. It's also the driest place on Earth, with extreme winds. Everyone who visits must undergo safety training to learn about dangers such as frostbite, hypothermia and snow blindness. Obeying safety rules is essential for survival.

"The point of the safety training is that Antarctica can be incredibly hostile," says Peter West of the National Science Foundation. "It can change from a relatively benign situation to that very hostile situation unbelievably quickly."

Additional training may also be required. Michael Gooseff, a University of Colorado professor who studies how ecosystems respond to climate change, regularly goes inland to the dry valleys. He has to take courses on how to camp in the ice, interact with helicopters, use the radio and rescue someone from a crevasse, a crack in the ice that can be hundreds of feet deep.

In 13 years, Gooseff has never experienced an accident, which he attributes to training and thinking ahead.

"We spend a lot of time talking about risk and the potential consequences," he says. "That mind-set really does help all of us to think about what we do before we do it."

There are also rules to keep Antarctica pristine. Nothing can be removed as a souvenir, and everyone has to properly dispose of their trash, down to ash from fires. People aren't allowed to approach wildlife, which include seals and penguins. The rule of thumb is that if the animal is reacting to you, you're too close.

Sometimes this can lead to strange occurrences. Anna Bergstrom, a hydrologist at the University of Colorado who studies how dust enhances melt on glaciers, occasionally sees seals and penguins wandering through the valleys, miles from shore.

"We're obviously, due to the treaty, not able to turn the penguin around and send it in the right direction," she says. "We just have to watch it, which is really sad."

Bergstrom frequently sees mummified seals in the valleys, as the cold, dry atmosphere preserves their bodies. Carbon dating reveals that some of the mummies are thousands of years old.

"They're all over the place, too," Bergstrom says. "They're up on the glaciers, and we don't know how they got up there or why they got up there."

It's one of many mysteries scientists still have to solve about Antarctica. With climate change melting the ice, their work is more important than ever. Increasingly, what happens in Antarctica affects the rest of the world.

"It's this gigantic landscape that's hostile to us, in the sense of being cold and windy all the time," Gooseff says. "Despite that, there are these beautiful hidden treasures to try to find in the form of scientific discovery."