Apple's annual developer conference in San Francisco.
Progress on Apple Campus 2 Continues as Walls Go Up
In the most recent shot, a wall has been built around the perimeter of the building, outlining where the foundation will be poured. A different tweet by @DawnMortensen notes that Pruneridge Avenue, a road that runs through the campus itself, has been permanently closed.
Street-level photographs back in December revealed several buildings still standing, but another aerial photo from February suggested a large portion of the demolition work had been completed. During construction, Apple has closed off sidewalks and built a large green privacy fence to keep the area hidden.