Hyperboria is the massive city from which the Oniss hail. The entire city was constructed atop a great shale of stone and around 20AF a tremendous earthquake sent half of the city plunging into the ocean. The remaining half of the city was unable to do anything to properly repair the city as constant raids made defense a central issue. Instead the sunken half was sealed off for a time.
Later Oniss used potions of water breathing to allow them to pump water from the sealed city.