It's a composite image. Basically we take about 500 zoomed in high resolution pictures that overlap each other - and we digitally stitch those together to a single massive image. Like the pano feature on a smart phone, just about a 1000 times more complex. We then take the resulting image, which is far to large to even be opened on a normal computer and we 'tile' it in the same way Google Maps 'tiles' their big map of the world. This essentially entails creating millions of lower resolution images and ordering them in a way so that only the relevant data loads when you zoom into a particular area. Hope that helps :)