The Wandering Sea is a living ocean in the Twilight Mirage. Originally, it was on the planet Quire but after the miracle, it floated in space around the planet Brighton.
The sea is a full ecosystem with a natural will to survive. It contains wildlife inside of itself, consisting of coral, seaweed, and plankton. This wildlife can be changed or replaced, but it needs all components to thrive as a being. When the Twilight Mirage was formed, its plankton died off without the sunlight it needed to survive.[1] In response, it flew off the planet Brighton, and tried to find a way out of the Mirage. It is around 5,000 square miles, about the size of Connecticut.
The Wandering Sea can be manipulated or exploited as a resource. Advent tried to capture the sea for months, in order to turn it into a predictive energy machine akin to Crystal Palace.[2] It was also attached to the Creeping Chain of the Herringbone Flotilla, literally chained by it's coral to pull it forward. Once The Notion unites the sea with its plankton, it's able to break through the Mirage and leave to a nearby sun.[3]