Inside Spheres, Amazon's massive urban gardens in Seattle

SEATTLE -- Amazon is preparing to open the doors to an interesting addition to its main campus: three massive, glass-covered domes.

They're called the Spheres, and they're essentially large greenhouses containing more than 40,000 different plants. The plant collection, which took years to cultivate, includes selections from all over the world.

The Spheres feature 4,000 square feet of green walls that alone contain more than half of the Spheres' plants. According to Amazon, the walls will help reduce the urban heat island effect, clean the air within the domes and naturally insulate the structure, which is climate-controlled using heat generated by Amazon's nearby data center.

Contained within are meeting spaces and an Italian restaurant and bar expected to open later this year.

While they are primarily intended for employee use, the 90-foot-tall Spheres do contain public retail areas and will offer public tours.

Amazon said the Spheres were originally inspired by biophilia, the notion that "humans have an innate urge to connect with nature."

"In an urban core, to have the ability to lose yourself in a little piece of nature is important, and it can have a great impact," Amazon horticulture program manager Ben Eiben said of the project.