Robert Smithson, Spiral Jetty, 1970. Photo © George Steinmetz/Corbis
Located on Utah’s Great Salt Lake, Spiral Jetty is Smithson’s masterpiece. Since its creation it has been weathered by the elements, and has recently come under threat from proposed oil drilling operations in Utah. The Dia Art Foundation currently owns the work and is leading the fight to preserve it.

Charles Jencks, Garden of Cosmic Speculation, 1989–2007. Image courtesy of Paulus Maximus, 2010
Considered the father of architectural postmodernism, Charles Jencks created his Garden of Cosmic Speculation at his mother-in-law’s house in Portrack, Scotland. The cascading mounds of earth are shaped to symbolize the evolution of the universe, from the human genome to the great equations of physics.

Aerial View of Line Drawing, Nazca, 1970–1997. Image © Yann Arthus-Bertrand/CORBIS
Among the world’s most puzzling art mysteries, the giant geoglyphs of the Nazca desert in Peru were inscribed into the earth between 500 BC and AD 500. Depicting stylized plants, animals and deities, they cover 450 square kilometers.

James Turrell, Roden Crater: Complete Site Plan, 1983. Private collection, Munich
James Turrell has been working on his Roden Crater since 1979. The earthwork occupies an extinct volcano and is designed to channel natural light for a unique, transcendental viewing experience. The crater is set to open to the public in 2011.

Christo, Surrounded Islands, 1983. Photo by Susan Greenwood/Liaison/Getty Images
Christo and his late partner Jeanne-Claude never allied themselves with the land art movement, but their wrapping of monuments and natural features have often toyed with geography. For Surrounded Islands, the duo swathed eleven islands in Miami’s Biscayne Bay with 6.5 million square feet of pink plastic.

Robert Morris, Observatorium, 1971–77
Designed to outline the rays of the sun at equinox in September and March, Robert Morris’s Observatorium was initially built for a temporary exhibition in Santpoort, the Netherlands, in 1971, but since 1977 has been permanently located in the country's Flevoland region.

Francis Alys (in collaboration with Cuauhtémoc Medina and Rafael Ortega), When Faith Moves Mountains, 2002
Francis Alÿs’s ambitious performance piece saw 500 volunteers shift a 1600-foot sand dune four inches from its original position, just outside of Lima, Peru.

Jim Denevan, Untitled (meeting place of two three-mile circumference circles, looking south, Nevada desert), 2009. Image courtesy Jim Denevan
Jim Denevan creates ephemeral works on beaches and dry lakes, drawing in the sand with found pieces of driftwood. In 2009 he produced the world’s largest freehand drawing—a three-mile wide work in the Nevada Desert that was washed away by a rainstorm the week after its completion.

Palm Island, NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
This pair of palm tree-shaped artificial islands on Dubai’s waterfront are part of an intended trio. The third is currently on hold as recession hits the city and it seems less likely that investors will come up with the rest of the $50 billion required to finish the development.