Visitors to Milan, Italy, may be surprised to see a forest towering above them. It’s actually an apartment building designed by an architecture firm that wanted to create a “home for trees that also houses humans and birds.”

It’s called the Vertical Forest and was built in 2014. Today, it’s covered in greenery. Plants grow in deep planters on each of the large balconies. They help control humidity, produce oxygen, and absorb carbon dioxide, reducing the buildings’ carbon footprint. They also help create a climate-controlled interior by filtering sunlight entering through the windows. A team of “flying gardeners” descend from the buildings’ rooftops to care for the gardens. Computers monitor and water the plants using the buildings’ irrigation system.