Nemausus (also called, dyslexically, Menausus) is, or was, the third layer of Arcadia. It completely slid into Mechanus only recently, thanks in no small part to the "reeducation" camps built by the Harmonium to forcibly excise all chaos from those they interned there. The stern, expansionist drive of the local Formians has also been blamed.
Nemausus has now been transformed into an enormous gear in the plane of Mechanus. There, Arcadian petitioners and other natives of Arcadia struggle against the Modrons and other beings who would make the realm a permanent part of the Clockwork Nirvana.
The cataclysmic nature of this change cannot be overstressed. Nemausus was an integral part of the spiritual evolution of Arcadian petitioners. The Nemausus petitioners, who resembled wise old men and women, migrated to Nemausus to live out their final centuries before acheiving perfect harmony with their plane. Now those who acheive that spiritual state are stranded in Arcadia's first two layers. Even the gods who once dwelled in Nemausus have been forced to move house if they wish to remain in Arcadia, as the foundations of belief upon which they built their realms has crumbled beneath their feat.
When people doubt that mortals can have a real affect on the shape of the multiverse, the example of Nemausus should suffice to set them straight. As it is, however, the truth about Nemausus is being kept a deadly secret by the Harmonium, who are trying to figure out what could have possibly gone wrong with their best-laid plans.
Sources: Planes of Law, 3e Manual of the Planes