Observation hole

Observation holes are devices used by Shinigami in the Shinigami Realm that allows them to observe the Human World.

Appearance
Observation holes tend to look spherical in shape and most are old and worn out as if they were hundreds of years old and have never been maintained.

Purpose
Observation holes are a necessity for Shinigami as they make it possible for them to find human victims without travelling to the human world themselves. If a Shinigami already knows the name, lifespan, and face of a human, they can use the observation holes to instantly locate and watch the human by thinking of what they look like.

These holes can be found all around the Shinigami Realm; there are even "desert areas with holes for looking down to earth. Nothing else is necessary."

Plot
Throughout the plot, the observation holes are rarely seen being used. The Shinigami Gelus uses one to watch a human girl he had grown attracted to. It is also used later on by Sidoh in an attempt to locate his lost Death Note.

Trivia

 * The devices' appearance is likely based on the Sfera con Sfera sculpture held at the Vatican Museum.