It’s hard to know what a sustainable building even looks like today, with a glut of different “green” classifications, LEED levels, and marketing pushes. But one criterion, the Living Building Challenge (LBC), founded in 2006, outstrips the others. And now its flagship building—a 50,000-square-foot office building in downtown Seattle called the Bullitt Center—is being called the greenest commercial building in the world. The $30 million building’s opening ceremony is today, a fitting date as the foundation’s president, Denis Hayes, is also the founder of Earth Day. Once tenants are moved in—educational facilities on the bottom and office space up top—the structure is expected to use only 23 percent of the energy typically required for a building of its size.