At first glance, a glacier looks like a rather lifeless thing—a barren chunk of ice. But there’s much more than meets the eye: Glaciers are home to a host of tiny organisms that make up a flourishing frigid ecosystem.

Most prominent among these in western North America are ice worms. Measuring about a half-inch in length and thin as threads of dental floss, ice worms (Mesenchytraeus solifugus) dot glaciers throughout the Pacific Northwest, British Columbia, and Alaska. Vast numbers of the tiny black worms emerge on summer afternoons and evenings to feed on algae, microbes, and other detritus on the surface. Then they burrow back into the ice at dawn—and during the winter, disappear into the frozen depths.

These distant relatives of