A hiker photographs the opening of a moulin—a tunnel that courses though the glacier—in the ceiling of a cave under the Mendenhall Glacier, June 16, 2014. Glacial caves are constantly changing; this cave collapsed a few weeks after this photograph was taken. #

Good crampons and stiff boots like these are required gear for safely hiking onto glaciers, July 29, 2009. Areas around crevasses and moulins are unforgiving: The ice is slick, about as hard as concrete, and accidents can be deadly. #

Ice climbers near the bottom of the ice falls on the Matanuska Glacier, July 22, 2016. During the summer months, guided ice-climbing trips—ranging from simple introductions to the sport to all-day, intensive courses—are available from local guides. #

Early morning in front of the Worthington Glacier near Valdez, July 3, 2016. This is the view from an observation deck that is just a short walk from a parking lot and a paved trail. #

The surface of the Matanuska Glacier is visible under a think layer of rock debris, known as a moraine, July 22, 2016. Glaciers are like giant conveyor belts for rocks, which they scour from the mountains up-valley and deposit in moraines as they flow. #

Jessica Taft pauses above the Harding Icefield, August 27, 2016. The ice field is thousands of feet thick, but it does not completely cover the mountains; those peaks that stick through are called “nunataks,” from the Inuit word for “lonely peak.” #

An ice “beach” along a supra-glacial lake on the Matanuska Glacier, July 2009. Lakes of melt water often form on glaciers; they can be stable and last for years or ephemeral, quickly draining when crevasses open under the surface. #

Helicopters ferry tourists above the Mendenhall Glacier for aerial views, July 26, 2012. Although several vistas are reachable by foot, many visitors opt to go up in helicopters—a quicker, if more expensive, option. As a result, there is a constant buzz of propellers in the Juneau area during the busy summer months. #

Bill Jirsa, a guide with Mica Guides, scouts for a route to the ice falls near the toe of the Matanuska Glacier, July 2009. The ice fall seracs—towering columns of glacial ice—are a popular destination for climbers. #

The glaciers don’t crush all the rocks they transport. Those that remain intact are deposited as the glacier retreats and are known as “erratics.” Erratics can range in size from enormous boulders the size of buildings to small boulders, like this one near the terminus of the Matanuska Glacier, July 29, 2009. #

The Harding Icefield has an otherworldly look, but it is just a day trip from Alaska’s largest city, Anchorage, August 27, 2016. The well-maintained and marked trail to the ice field is a steep four-mile hike (one way), but it is nonetheless very popular in the summer. #

A hiker (bottom right) is dwarfed by the massive, heavily crevassed ice fall where the Harding Icefield begins its descent into Exit Glacier, August 27, 2016. The Harding Icefield is the source of some 30 glaciers on the Kenai Peninsula, though the Exit Glacier is the most accessible. #

Tourists explore the lower section of the Worthington Glacier, July 3, 2016. Because the glaciers are so accessible, many people wander onto them without the proper equipment, such as helmets and crampons. #

An aerial view of the Harding Icefield looking toward the Exit Glacier trail, January 18, 2012. The Harding is one of only four remaining ice fields in the United States, and it is the largest that is completely contained within the U.S. border. #

A guide uses crampons to climb over a moulin on the Mendenhall Glacier, June 16, 2014. Moulins form when melt water and runoff find small cracks and depressions in the glacial surface and erode the ice, creating tunnels. The moulins can be dangerous and extremely deep, leading into the internal plumbing of the glacier. #