Massive glaciers can take hundreds or thousands of years to form, but climate change has been destroying glaciers in a matter of decades. In 1850 the area in Montana that eventually became Glacier National Park contained about 150 glaciers. Because of global warming, only about 25 still exist today. Jackson Glacier, photographed in 1912 and again in 2009, is approximately the seventh largest of these remaining glaciers. Scarring and scraping on the exposed rock reveal the glacier's former reach, and vegetation now grows where ice once flowed. Using either your mouse or arrow keys, drag the slider to reveal the glacial retreat caused by climate change.