In the 1970s, around the time of the first Earth Day celebrations, artists such as Robert Smithson set out into the great American West with bulldozers, eager to redefine mankind's relationship with the natural world. They made massive marks on the landscape—digging giant holes, piling up mounds of soil, even dumping asphalt down hillsides in the desert—an art form akin to the great earthworks of ancient civilizations. Smithson, who died in a helicopter crash at the height of his career, became the beloved godfather of the genre known as land art; his 1970 masterpiece, a 457-meter-long curlicue that stretches out into Utah's Great Salt Lake and now spends most of its days underwater, has become the movement's trademark.