The Rift Plane is made up of floating rock formations lit by candles. It is very dark blue and the ground is littered with seashells, giving it the impression of being underwater despite floating in the air.
It is where Neu Hiraleon ended up after being sucked into a dimensional rift created by Dr. Brahman's tampering with the gravity engine. The Nevi living here eventually take control of it and turn the entire city into something resembling a living organism.
The Council of Jirga Para Lhao planned to mine a great deal of ore from here, and therefore kept the rift to this plane open by firing at it. However, they inadvertently allowed Neu Hiraleon to get through and obliterate the entire Aravash au Governa island.