FLAGSTAFF, Ariz. — Flying a couple of thousand feet above a volcanic field in Arizona near the Painted Desert, it's fairly easy to spot the extinct volcano known as the Roden Crater.

It stands alone in the field, apart from hundreds of other volcanoes. It shows signs of being a manmade monument, with paths winding around it, a small building with the horizontal thrust of a Neutra home embedded in one side and an entrance nearby.

And down in the center of the crater's bowl, a 44-foot-wide concrete ring surrounds a large hole — looking up like a giant, unblinking eye.

FULL COVERAGE: 2013 Spring arts preview

You can imagine people thousands of years from now puzzling over its function, much the way people today wonder about Machu Picchu or Stonehenge. Was the eye of the crater a symbol of political power or a site of a religious ritual? Was the plaza surrounding it a stage for some sort of sacrifice?

As contemporary art disciples today know, the Roden Crater is actually an epic artwork in progress by James Turrell, who turned 70 this month and has a snowy white beard to go with it.

Turrell has devoted nearly 40 years and a good part of his income to the crater, and he's completed construction on only about one-third of his plans. The goal is to transform the crater into what he calls "a naked-eye observatory" for viewing astronomical events.

He has moved tons of earth to subtly reshape the crater rim and bowl. He has built assorted chambers and tunnels inside for viewing celestial events like sunsets, solstices, solar eclipses and lunar standstills, framing them as images worth contemplating every bit as much as a starry night by Van Gogh.

In this way, the astronomical observatory doubles as art installation, designed to sharpen our patterns of attention and perception.

PHOTOS: Arts and culture in pictures by The Times

"You could say I'm a mound builder: I make things that take you up into the sky. But it's not about the landforms," said Turrell, known as a pioneering Light and Space artist as opposed to an earthworks artist like Robert Smithson or Michael Heizer. "I'm working to bring celestial objects like the sun and moon into the spaces that we inhabit.

"I apprehend light — I make events that shape or contain light."

He also has some more worldly events on his mind these days. On May 26, a much-awaited retrospective of his work opens at the Los Angeles County Museum of Art. Also this month Turrell's L.A. gallery Kayne Griffin Corcoran is opening a new space on La Brea, designed by the artist, with a show that documents the evolution of the crater from early plans in 1976 to today. It includes photographs, drawings, blueprints and models as well as actual tools used in the process — such as his aerial camera.

The following month, Turrell takes over the Guggenheim Museum rotunda in New York designed by Frank Lloyd Wright, creating a site-specific work using colored LED lights along with natural light from the skylight. Also in June, the Museum of Fine Arts, Houston, opens an exhibition showcasing his works, large and small, from its collection.

Alison de Lima Greene at the Houston institution said Turrell has been making important museum installations since his breakthrough Pasadena Art Museum show in 1967, when "he was the first artist to put on public display an exhibition where light was disembodied from any objects."

CHEAT SHEET: Spring Arts Preview

These installations, she said, "allow viewers to reconnect with the joy of pure perception and the beauty of pure light."

But the crater itself is very much in progress and closed to the public. Early on, Turrell announced that the crater would be finished by 1990, but funding dried up and his plans grew only more ambitious.

Later, the target for opening it to the public became 2000. It was not to be. Finally, the last time Turrell or his supporters went on record talking about a completion date, the goal was 2011, when the LACMA show was first scheduled.

But construction on new areas has lagged, and one major project — an 854-foot-tunnel designed to deliver multiple perceptual effects — suffered delays after contractors allegedly cut corners.