Monumental Land Art of the United States
Land Art, also called Earth Art, Environmental Art or Earthworks, is primarily a sculptural movement encompassing creative work that integrates physical or conceptual elements of the landscape into the finished piece. The most famous of these are often known as much for their monumental scale as for the intrinsic qualities of the artwork itself.

Listed below are links to satellite and topographic imagery of the best known large earthworks, both old and new, still extant in the continental United States, as well as a few significant smaller scale pieces. The images provide a sense of comparative scale; the topographic views provide a sense of the terrain; and the geophysical coordinates provide a sense of their proximal relationship.
(Via The Long Now Blog)