These images were collected as part of an ongoing joint project between the U.S. Geological Survey and NASA, known as Landsat. Their companions make pictures of Earth from space since the 1970's. All the images are sent to Earth and archived to tape drives USGS, which look something like this:
Google in 2009, began working with the USGS on the digitization of the archive images. With the technology of Google Earth Engine, they went over 2,068,467 images - a total of 909 terabytes of data - to select the highest quality (for example, no clouds), for each year from 1984, and for each place on Earth. Then they gathered them into huge planetary images by terapikseley 1.78 each, one for each year.
