The National Parks Service has put together one of the more interesting photography projects I've seen recently with its comparison of photographs of Glacier National Park and Yellowstone Park from the 1930s to today.
The parks service has digitized photographs shot about 80 years ago and has merged them with photos shot in the same locations today. The photos were shot from fire lookouts, and the modern photos were shot with infrared film to eliminate effects from pollution.
At the National Parks Service web site you then can see the full photos with the old portion on the left and the modern portion on the right. You can use a slider bar to change how much of each portion is visible, or you can download a high resolution version. Looking at the differences is pretty interesting. Just click on the link above to see the photos in the project.
Photo courtesy National Parks Service