In 1974, shortly after he was awarded a Guggenheim Fellowship, James Turrell spent seven months crisscrossing the vast Arizona desert in a small plane, searching for a venue for the most ambitious artwork of modern times. Spotting an extinct volcano almost two miles wide, he landed at its red-dirt base, climbed to the top of the cinder cone, and unfurled a sleeping bag. That volcano became the site for his magnum opus, a huge naked-eye observatory called Roden Crater, which aligns a series of chambers, tunnels, and staircases with celestial objects and events. Despite spending half a century shifting over 1.3 million cubic yards of earth, however, it still isn’t finished. Instead, the artist has completed 99 Skyspaces—single-room versions of Roden Crater—in locations around the world. Turrell, now 83, unveils number 100 in Aarhus, Denmark, at ARoS. A huge grass-covered dome with a central oculus, the work is his largest ever museum Skyspace. —Harry Seymour