"Navigate below ground
Because Ground represents the surface of the earth, you can apply navigation rules to it. By default, navigation below ground is disabled. It is often unnecessary to go below ground and can be disorienting when you go there by accident. If, however, your scene contains data that correctly belongs underground—such as subsurface utility pipes or geological features—you can enable below-ground navigation. In this mode, the height list in the lower corner of the scene view shows negative values when you're below the ground surface. You can enter negative numbers directly, such as -100m, to navigate the camera vertically below the surface.
To enable navigation below ground, follow these steps: