The Tunnels are a designation of the Guardians’ traveling network by other species, often tainted with superstition.

In actuality, they are using the knowledge of the connecting fabric between the planets and making use of it to imprint it with passageways that can retain more or less the imprint, depending of the frequency of usage of the passageway.

Tunnels are different from the traveling portals in that they were of exclusive use of the Guardians and those able to manipulate reality as they did. They are believed to have almost entirely disappeared (resorbed) by the time of the Warring Kingdoms, as Dragons and Nirguals only seldom made use of them.

His gaze became somber. The fear of some caves was deeply buried into the psyche of most people living in this world. A fear inspired by the Guardians, as caves often were passageways to those other worlds. And they were very useful.
(in ”The Caves”) 

• see Key of Ghört