Spiral Jetty by Robert Smithson
"Spiral Jetty" is probably one of the most famous pieces of Environmental Art in the world. Also called "land art" or "earth art", this installation was built in 1970 from bulldozed mud and rocks to form the spiral out into the Great Salt Lake in Utah. (official site)
Google Maps and other online mapping applications have made art like this far more accessible to people.
See also my blog post on the subject, Earth Art Collection From Google Maps.