The atmosphere beyond the aetherosphere. No one knows what it is made of, or if it is even breathable.
- "Rue wondered, not for the first time, what the aetherosphere looked like from above. And what, in fact, the layer above the aetherosphere was made of-was it even breathable? She was not alone in this curiosity-aether scientists discussed the outersphere as if it were a desirable undiscovered country, and were always concocting new ways to go higher. So far, however, no one had managed to break through." (Prudence, Chapter Five)