A gravity globe, or grav globe, was a human device to provide massive illumination. It could be sent aloft under its own power so that it would 'hang' at a set height and provide light in a large space. Gravity globes were particularly useful in large dark caves.
Ida Scott used a globe to light up Krop Tor's core. (TV: The Impossible Planet/The Satan Pit)
The Clerics used a globe to light up the Maze of the Dead. (TV: The Time of Angels/Flesh and Stone)