How Google Earth Works
"...[H]ow is it possible for you to zoom in from outer space to a point somewhere above the rooftop of your house without bringing your desktop computer to a grinding halt?
...If one were to download over the Internet a one-metre resolution image of the entire world it would take 69 years with a 10-megabit-per-second Internet connection, and 12,400 years with a standard 56K modem.
To slash the amount of data they have to transmit across the Internet, virtual globes such as Google Earth approximate the sphere of the planet's surface with a polygon made up of flat tiles. The further away your viewpoint is from the surface, the fewer tiles are needed to create the illusion of roundness, and the lower the resolution of these tiles can be."