South Lawn
The two fields between Butler Library and College Walk. Almost always (or partially) closed, as indicated by the red flags. In the winter, they're usually concealed by white tarps to protect them from the elements, so you can't even see them. Honestly, you can't make this stuff up.
Before the acquisition of Baker Field, the area was used for athetic purposes. A football field was set up in the fall, and Lou Gerhig played baseball here as well.
According to legend, royalties from the film Ghostbusters pays for the lawn's maintenance.