Just outside Cordova, Alaska, hulking walls of ice loom like white ghosts. Five glaciers—the Scott, Sherman, Sheridan, Miles, and Childs—creep down from the Chugach Mountains toward the town, on Prince William Sound. From afar, they look clean, but up close the pockmarked ice is embedded with particles of dirt, boulders, branches, and dead insects. Sheer crevasses, seemingly bottomless, slice through them. When the air is warm enough, the glaciers smoke like volcanoes, and the fog rising off their jagged edges settles over the Copper River flats.

"Glaciers are delicate and individual things, like humans. Instability is built into them." —Will Harrison, glaciologist

The flats themselves, a vast wetland and annual rest stop for millions of shorebirds, are the result of thousands of years of glacial-silt buildup. More than mere earthmovers, glaciers are epic travelers, transient masses of ice that migrate almost imperceptibly but can flatten mountains, create lakes, even orchestrate geological ages. In Alaska, they cover some 29,000 square miles, or 5 percent of the state's considerable territory. More than three-quarters of Alaska's freshwater is held in glaciers, and most of its major rivers spring from them, including the Copper, where the salmon are marathoners, strong from the 300-mile swim to their spawning grounds. The Copper also carries blocks of ice that have split from the Miles and Childs downstream to the Gulf of Alaska. The river flows between the two glaciers, undercutting them and causing bergs to break off—sometimes hunks the size of houses. Crashing into the river, they can create waves so huge they've been known to hurl salmon into nearby treetops.

As powerful as they are, the fivesome near Cordova are now in retreat. The Miles alone has shrunk by more than 7,000 acres since the 1900s. (In 2002, scientists reported Alaska's glaciers were thinning at double the rate of the previous decade, likely because of global warming.) The glaciers of Cordova are imperfect, scarred and sometimes broken. But while they last, they look like the purest things in the world.
—Ashley Shelby