The helicopter heads out over McMurdo Sound, crossing a monotonous expanse of sea ice broken only by a smattering of icebergs and occasional clusters of Weddell seals. At last, off to one side, you see it: the Ross Ice Shelf, a mesmerizing expanse of white that stretches to the horizon and beyond. Wreathed in ice fog, the ice shelf takes on the haziness of a mirage. Yet it is all too substantial. Its surface ripples with undulating pressure ridges and solid, wind-hewn waves called sastrugi that move with the ice as it flows inexorably toward the sea.

Like everything else in Antarctica, the Ross Ice Shelf is outsize, a chunk of frozen water bigger than Texas jutting off a continent that is half again as large as the U.S. It is deeply mysterious, for its ice, like the ice that covers the rest of Antarctica, conceals far more than it reveals. Three years ago, the Ross Ice Shelf started calving icebergs so big that they invited comparison with Massachusetts and Connecticut, and some of these bergs — including C-19, which broke off the shelf last May — lurk nearby, provoking consternation and wonder.

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What does this sudden flood of icebergs mean? Was the ice shelf, as most scientists think, responding to its own internal rhythms of expansion and retreat? Or was it reacting, through some connection in the ocean perhaps, to the general warming that has taken place almost everywhere else?

A quarter-century ago, such questions would not have seemed terribly urgent. But now that the earth is incontrovertibly heating up--2002, climate experts report, edged out 1999 as the second warmest year on record, after 1998--concerns about the overall stability of Antarctica's ice are on the rise. And with good reason. Locked away in that ice, after all, is 75% of the earth's freshwater, enough to raise global sea levels 200 ft. And while no one expects all that ice to melt anytime soon, a significant portion — enough to swamp low-lying coastal zones and menace major cities from Bombay to Boston — conceivably might.

Within the past year, scientists watched in awe as a giant ice shelf disintegrated in the Antarctic Peninsula in just over one month's time, and in a remote region of West Antarctica, satellites have detected an expanse where glaciers are worrisomely speeding up their transport of ice to the sea.

Yet it is hard to gauge what these dramatic developments portend, for despite scientists' best efforts, Antarctica — the highest, dryest, coldest continent on the planet — remains a climatological cipher. For example, while it is clear that the Antarctic Peninsula — a thin sliver of land that juts above the Antarctic Circle — has been rapidly warming, the vast empty spaces of East Antarctica, repository of the greatest ice sheet on earth, appear to be doing the opposite. "Here we have a continent that is so important to our future," says earth scientist Peter Doran of the University of Illinois at Chicago, "and we can't even agree on what's been going on there for the past few decades."

In fact, the most basic questions — Is Antarctica as a whole warming or cooling? Is its ice cover thinning or growing?--cannot yet be answered definitively. For one thing, the continent is too big and measurement points are too few and far between. Also, scientists lack the long-term records needed to put the present in perspective.

This situation is changing, however, as researchers begin systematically to probe Antarctica's ice, rock and sediments for clues. Their work is always arduous, often heroic and sometimes dangerous. This was brought home two weeks ago, when a helicopter shuttling supplies to a scientific field station crashed near McMurdo Sound, injuring two.

TIME visited the region to see the continent close up and assess the state of science there. What we found was a forbidding and extraordinarily complex environment that is just starting to come into focus. Some highlights from our journey:

Marscape: Mcmurdo Dry Valleys
That Antarctica is so little understood is not surprising, for it is a remote, otherworldly place that in many ways resembles early Mars more than contemporary Earth. And no place is more Martian in character than the McMurdo Dry Valleys, a wedge of rugged, rocky terrain stippled with ice-covered lakes and overhung by glaciers. No diminutive alpine plants cling to the slopes of these valleys. No rodents scurry amid the boulders and scree. No flies or mosquitoes whir through the air; no fish, mollusks or crustaceans dwell in the lakes and streams.

"A valley of the dead," declared explorer Robert Falcon Scott after discovering Taylor Valley in 1903, and at first glance it would seem he was right. Yet there is life in the Dry Valleys, albeit life that is primitive in form and exceedingly cryptic. Minuscule roundworms called nematodes and insects known as springtails constitute what biologists jokingly call the "lions and tigers of the soil." The top of the aquatic food chain is occupied by single-cell protozoa that feed on bacteria.