When the Pathfinder chapterhouse and library (what became known as Bibliothèque Perdue) was destroyed, something happened to the catacombs beneath the library. Nobody knows exactly what happened, but starting perhaps a hundred feet beneath the old site a chasm appeared. Thirty feet across, the chasm (which causes an intense feeling of dread in even the most hardened individual) descends into stygian darkness beyond the sight of even the best vision. Torches disappear, objects with light spells fade and vanish into the gloom; those who dare to venture below never return.
Although the chasm was once as smooth as limestone can be, weathering and the general increase in decay the area seems to possess has pocked the walls, creating cracks and gaps and access points.
L'Abyss as an origin / destination is considered 'Logical'.