Roden Crater, Ongoing
This is quite literally a crater—a dead volcano outside Flagstaff, Arizona, that Turrell acquired. Since 1974, he's been working on this project, a kind of a naked-eye observatory focused on the viewing and contemplation of celestial bodies. It's considered the world's largest art piece and an admirably ambitious work, at times compared to Stonehenge and the Mayan Pyramids. Critic David Cohen has described it as such: "a multimillion dollar earthwork of tunnels, elliptical chambers, and a perfected rim—it is designed for enhanced contemplation of the heavens, a kind of natural telescope that will use the laws of perception in lieu of lenses."