From the project website (accessed 2019-07-16):
CORONA is the codename for the United States’ first photographic spy satellite mission, in operation from 1960-1972. During that time, CORONA satellites took high-resolution images of most of the earth’s surface, with particular emphasis on Soviet bloc countries and other political hotspots in order to monitor military sites and produce maps for the Department of Defense. The more than 800,000 images collected by the CORONA missions remained classified until 1995 when an executive order by President Bill Clinton made them publicly available through the US Geological Survey. Because CORONA images preserve a high-resolution picture of the world as it existed in the 1960s, they constitute a unique resource for researchers and scientists studying environmental change, agriculture, geomorphology, archaeology and other fields.