According to PSU geography and geology professor Andrew Fountain, the melting of alpine glaciers in particular is considerably affecting the planet's sea levels. "These guys are melting like crazy," says Fountain, whose research team studies glaciers throughout the American West. "Right now, they're making the most significant contribution to sea-level change, other than thermal expansion of the seawater."

Although at present the planet is growing warmer, studies conducted in Antarctica by Fountain's team have found that glaciers at the bottom of the world are neither growing nor shrinking. "They are in wonderful equilibrium," Fountain says. "[But they're] kind of the exception to the rule."

Those in Antarctica aside, the vast majority of the world's tens of thousands of glaciers are undeniably receding. Here in the United States, glacial melting is an accepted fact. A new study from the National Climatic Data Center indicates that 2006 was the nation's warmest year in history. Glacier National Park in Montana has only 27 glaciers remaining out of approximately 150; it's estimated that by the middle of this century, nearly all the park's glaciers will be gone. Some studies are even predicting that by 2100, ski season in the United States could run only from Christmas to President's Day, with that being the best-case scenario.

Which is why I'm here at White River. It's easy enough to view a glacier from a plane, but some part of me wants to see one up close, within the confines of the Lower 48. I want to feel the cold under my boots and descend down into the belly of an ice mass hundreds of years old - before it disappears into a photo archive.

BY THE TIME my glacier-expedition posse meets, at 4:30 a.m., the other groups have already departed for the summit. My guide, Jon Bates, another TMG employee, double-checks my gear and hands me a helmet lamp.