Star Fissures can spawn in a Mystcraft age. They have the appearance of a fissure in the ground with no bedrock, going into void.
They teleport you back to the Overworld spawn if you jump into them.If you forget your Linking Book, or have no way out of an Age, you must find a star fissure to leave. Otherwise, you will have no way of leaving the Age. (Thus, making it impossible to leave legitimately.)