A Star Fissure is a phenomenon added by Mystcraft. It is a rift in time and space in a Mystcraft Age which tears a hole from the surface of the world through bedrock. At the bottom of this rift is a starry expanse, and anything that passes through will end up on the surface of the Overworld.
Star Fissures are the only non-cheating 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 near the age's spawn point. Using a minimap, fissures appear as transparent holes on the map, making locating a fissure very easy. However, since players often cannot see the stars at the bottom of a fissure from the surface, Star Fissures can be difficult to identify and find in Skylands ages.
A Star Fissure will usually return the player to within a few hundred blocks of the Overworld spawn point. After falling through a Star Fissure, the player drops from the top of the world, often dying from the fall unless falling into water, capable of flying, or protected from fall damage. This death should return the player to the Overworld spawn point. However, if the difficulty setting is set to Peaceful mode, the Star Fissure will return the player to the Overworld spawn point without dropping the player from the top of the world.
If the player is in an age that has no Star Fissure, one method is to find Sugar Canes, wood, and Feathers somewhere in the new age to make another Descriptive Book in an attempt to move to an age that does have a Star Fissure. When doing this, it is generally a good idea to stock up on supplies before moving as some ages do not have these materials readily available. Trapped players can also return to the Overworld by using NEI to place a Nether portal, go through, and then go back through.
If a player accidentally places a star fissure in the Overworld using NEI, it may cause future errors or crashes. In order to fix this problem, Lava can be poured on top of the fissure, which will delete it permanently from the Overworld.