Portal to the Human World

There is a portal in the Shinigami Realm that the Shinigami can pass through in order to enter the Human World.

Appearance
The portal looks like a hole at the end of a tunnel; it is bright and emits white light.

Plot
Ryuk first drops a Death Note to Earth through the portal, where it is found by Light Yagami. Five days later, Ryuk travels through the portal himself to meet Light.