TWO HUNDRED THOUSAND TONS of freshwater go into making your bigger Antarctic icebergs. They're formed when ice calves off a glacier or an ancient frozen shelf. Splashing into the salt water of damned-cold seas, icebergs drift off on their own, sometimes covering ten miles a day. Less dense than salt water but not by much bergs float like pear-shaped tubbies at the Jersey Shore, with 10 to 15 percent poking above the surface and a great rumpy mass below.

At the moment, a medium-huge berg roughly the size of a large yacht is minding its own business in Waddington Bay, a body of water off the north side of the Antarctic Peninsula, that spindly geographic thumb that points toward South America. Suddenly, an intruder starts hacking into its blue-white mass with a pair of ice axes. The blades belong to 39-year-old American Chris Davenport, a former World Extreme Skiing champion who's now a multi-sponsored, globally known ski mountaineer. Davenport swings his ax points into the ice. He kicks his way up it wearing ski boots. Hauling a pack with skis, he spiders up a wall that angles steeply into the bay.

Before long, he reaches the berg's 100-foot "summit," detaches his skis, lays them down carefully, and clicks into his bindings, knowing that it's not necessarily smart to ski an iceberg. Davenport's body weight and gear are putting 200 new pounds on top of its unstable apex. He and his fellow ski adventurers Australian Andrea Binning, 33, and Norwegian Stian Hagen, 35 have watched from afar as a few top-heavy bergs actually flipped over, with a thunderous splash. Sometimes the bergs simply implode and sink. The sounds of disintegrating chunks are echoing freely throughout the bay, and Davenport will tell me later that when he skied this berg, which he did twice, he could hear it groan.

So why is he doing it? The answer isn't much more enlightening than the old dog-licks-self truism: because he can. One skis icebergs in Antarctica precisely because you can't ski them in Verbier or the Khumbu. But the stunt is really just garnish to this trip's main event: skiing several of the dramatic, snow-covered spires that erupt out of the hyper-clear waters around here.

"The Antarctic Peninsula is the last great ski location on earth," says Davenport, who ought to know. In 2007, the Aspen-based athlete became the first person to ski all 54 of Colorado's 14,000-foot peaks in a year. Along with Hagen who cut his teeth on the icy couloirs of Chamonix Davenport has also knocked off Europe's most intimidating ski descents, including the Eiger, the east face of the Matterhorn, and Mont Blanc.

The basic plan this time: float in from Ushuaia, Argentina, on a 75-foot sailboat called the Australis, ride inflatable, outboard-powered Zodiacs to rocky beaches, climb peaks, and then relish descents that have probably never seen skis, moving cautiously because of the scary absence of paramedics and rescue helicopters.

"People have scoured the globe for the next Valdez," Davenport says, but almost no place can match Alaska's combination of huge peaks, maritime snowfalls, and opportunities for untouched lines. Antarctica just might. "The Antarctic Peninsula is like the Southern Hemisphere's answer to AK," he adds, "even though it's a lot harder to get to."