The ruins of Fathomir as shown on the cover of Kingdom of the Deep.
Fathomir is the place where the Seazarein lives and rules over the sea (unknowingly to most creatures). Fathomir is first mentioned in the third book, Into the Abyss, when Gray goes there and becomes a pivotal location moving forward for the rest of the series. This is also where The Battle of Fathomir takes place. It consists of a large open space with ruins of an ancient human civilization, a large cave where the Seazarein and his/her employees reside, and it's infamous golden greenie fields.
Location: Northern Pacific Ocean (near the bering strait)