"Sometimes," she says, in a distant voice, gazing at the wall, "the mind rejects Liminality. Individuals fall out of ambit. It requires more energy than we care to spend to retrieve them while their minds are still in rejection, so we leave them there. In time, sometimes, their minds begin to reach out again, so that they may become retrieved."

She looks back to Riddick. "You Travelers call it Dungeons."