The Sea of Khazak is a body of water in the central part of the Great Hunting Ground, likely somewhere in what is now western Asia. It might be the Caspian Sea. It is widely considered extremely dangerous for Traction Cities, being surrounded by the treacherous Rustwater Marshes. Anna Fang refers to a song or rhyme which is apparently widely known among the cities: "Beware, beware of the Sea of Khazak, for the town that goes near it will never come back."
The Sea of Khazak appears in Mortal Engines.