On board National Geographic Explorer heading down to Antarctica, an expedition guide briefed passengers on their next few days. "You will not be able to describe to your friends back home what Antarctica is like," he said.

"He was right," said Pat Henry, a University of Hawaii professor. "Antarctica is indescribable. It's pristine, the way the world was before people."

For Henry, one of 140 passengers aboard the small-ship cruise out of Ushuaia on South America's Tierra del Fuego, the journey to the northernmost tip of Antarctica was the chance to fulfill a dream.

"I'd always wanted to go to Antarctica; I even applied to work there one summer as a student but didn't get accepted," he said. "Getting to see it finally was life-changing."

It's a dream shared by many. Destination Antarctica is a once-in-a-lifetime opportunity for travelers, rated among the top places to see in 2010 by The New York Times travel staff.

Summer days are 20 hours long on the Antarctic Peninsula, one of the continent's richest breeding grounds for seabirds, seals and penguins.

Seasonal ice, not clocks and calendars, set the day-to-day schedule. And for those who can absorb the hefty tour costs, Antarctica has never been more accessible.

In fact, The New York Times has reported that 2010 may be the last year that Antarctica is open for mass tourism "not because the ice is melting too fast (though it is), but because of an explosion in the number of tourists to the world's last great wilderness."

According to the International Association of Antarctic Tour Operators, visitors have quadrupled in the past decade. About 46,000 tourists visited in 2007-2008, and 38,000 in the 2008-2009 season. Countries that manage Antarctica say the boom poses a major environmental threat and want to limit the number of tourist ships that visit during its short summer season — between October and April.

LIVING LABORATORY

No one owns the Great White South, not governments or people. The massive ice plateau is protected by the Antarctic Treaty, a series of agreements eventually signed by 47countries in 1961 that set aside Earth's fifth-largest continent for peace and science.

For scientists, the continent is an immense living laboratory the size of the U.S. and Mexico combined. For travelers, it's a haunting landscape of icebergs, glaciers, reflections and wildlife that provide adventure without the hard work of climbing Kilimanjaro or crossing the Sahara.

And, refreshingly for many, there are no tourist shops, no admission fees, no traffic. As remote from civilization as it's possible to be, it's nature in its purest form.

STUNNING VIEWS

Getting to the Antarctic Peninsula, which juts north toward South America, is by way of the Drake Passage, a ferocious stretch of water south of Cape Horn. Force 10 gales are common, and the seas can be huge — conditions tagged by sailors as "the Drake tax."

On Henry's crossing, however, the 600-mile passage was kind and two days after leaving Ushuaia, he and fellow passengers stepped ashore at Greenwich Island, the gateway to one of the planet's most stunning landscapes, a vast horizon, and the company of seabirds, penguins, and seals.

For the next seven days, the Explorer cruised along the east coast of the peninsula to within 65 nautical miles of the Antarctic Circle, enjoying the spectacular wilderness, the sight of rare Arnoux beaked whales and humpbacks.

For Henry, the proximity to the wildlife was a highlight.

"Each day someone would ask, 'How cute can the penguins be today?' Each day they were cuter," he said.

"We were in a place where human beings had had almost no impact. And because there are no land predators, the penguins are completely unafraid."

Talks by the ship's expert naturalists ranged from descriptions of early explorers whose obsessions made them risk their lives in small boats, to global warming and why the southern continent's weather system makes it more frigid than the Arctic.

In the evenings before dinner, guides recapped what passengers had seen that day.

While close attention to ice determined the Explorer's ultimate route, conditions favored passengers landing daily for ice walking, photography, kayaking or simply to be charmed by the armies of penguins and honking seals.

On Deception Island, passengers swam in the island's steamy volcanic hot springs.

"Hiking on Booth Island, and looking out over that landscape, sometimes with no one in sight, it was easy to imagine how it would feel to be the only person in the world," Henry said.

"It was such a calm feeling, of being completely at one with nature," he said. "We should leave Antarctica exactly as it is."