Aetherholes are where the mirajin has torn through the substance of reality. They are gateways to other places with mirajin energy, generally to other random aetherholes in Valenth. A few of these aetherholes are a window into the Reverie. However, these are very rare, and only a very small percentage of the aetherholes in Valenth lead to the Reverie.
When there is more than one aetherhole in an enclosed space a pocket of dreamspace is made.