Built in the late 1980s and completed in 1991, Biosphere 2 was designed as a sealed, self-sustaining ecological system in the Arizona desert. There are actually two lungs on the property, and visitors who opt for the add-on Lung Tour get to see these mechanical giants up close. The Lung Tour is widely recommended by those who have done it, and for good reason. Considering that this entire structure sits in the middle of the Arizona high desert, the contrast is nothing short of extraordinary. For those who want to go deeper, add-on tours including the Lung Tour and the library tour offer access to areas not covered in the standard self-guided experience.