Well, apparently there is a trick that makes finding the fissures easier
Star Fissure - Technic Pack Wiki
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 near the age's spawn. 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.