Construction on Apple's second campus in Cupertino, California continues to progress as its completion date looms, and this month, further work has been done on the main ring-shaped building that spans 2.8 million square feet. Structural framing is nearing completion, and construction workers are preparing the solar panels that will be installed on the roof. More glass window panels have gone up, along with window canopies in some spots.

The large, curved sheets of glass that surround the building are designed to let employees look out over the campus, which will be covered in greenery. Apple is planning to include 80 percent green space on the campus, with a central garden, orchards, and several outdoor dining areas.

Apple CEO Tim Cook recently said the large mound of dirt in the center of the campus will not be removed, instead going towards landscaping that will support more than 7,000 trees. "Hard to know which is more beautiful, the building or that pile of dirt," he said.

The roof of the underground auditorium, where Apple will host events, is now in place, and work on a 100,000 square foot fitness center is underway. Solar panels are being added to the second of two parking garages, which will provide parking for more than 10,000 employees. Underground tunnels are in place for access to underground parking and other campus areas.

Construction on the campus is scheduled to be completed at the end of 2016, and Cook believes Apple will begin occupying the building at the beginning of 2017.