An image of a Star Fissure.
A Star Fissure is a rift in time and space, that can only appear inside an Age. It manifests itself as a hole in the surface of the world, that drops down to the very bottom (bedrock). At the bottom of such a rift is a starry expanse and anything that passes through it, will end up on the surface of the Overworld at the original spawn point for that world.
Star Fissures are the only option for trapped players to return to the Overworld, though they are quite rare. Only ages that have the Star Fissure symbol will contain them, and it will show up close to the age's spawn.
The entrance to a Star Fissure
Since players often cannot see the stars at the bottom of a fissure from the surface, Star Fissures are difficult to identify and find in Skylands ages. Using Rei's Minimap, fissures appear as transparent holes on the map, making locating a fissure very easy.