Nadir

The point directly below the person viewing or below the camera. The opposite (above) point is named Zenith.

The Nadir is streched to a line in Equirectangular Projection hence it is hard to edit it directly. However, a rectilinear view can be extracted for editing.

The Nadir usually gives much more problems than the Zenith because there either is a tripod to cover or there are Parallax errors to edit. An usual way to solve this problem is to use Tripod Caps.