In 1932 visitors to Glacier National Park, Montana, could walk beneath a massive arch of ice at the Boulder Glacier ice cave. By 1988 the glacier that created the cave had vanished completely, leaving behind only bare rock and gravel. This striking photo pair helped inspire the U.S. Geological Survey (USGS) to begin documenting glacier and landscape change through photography. Using either your mouse or arrow keys, drag the slider to reveal how much Boulder Glacier has retreated.