The Shard Realm of Correspondence was once heavily studied by the Ahl-i-Batin, before the settlement on Mercury was mysteriously destroyed. Reports from manuscripts salvaged by the refugees say that it is a very difficult place to understand, with at least five spatial dimensions and two others.
Stories say that it's entryway from the Horizon appeared to be a massive, flat plane with its broad side always to the viewer. No one ever found an edge or was able to put a volume measurement on it. Those with an understanding of correspondence could decide to enter the portal or just pass through it to the other side. The entrance had no measurable thickness.
- Beyond the Barriers: The Book of Worlds, p.141-142