Mashu was a mountainous region in Mesopotamia, located a week's journey from Uruk.
After Utnapishtim's space ark was damaged in the battle with Qataka, it was forced to make an emergency landing in the mountains of Mashu. To avoid contact with the natives, the ship was hidden under an island in the middle of a lake in the crater of an extinct volcano. Further, the lake was stocked with a species of killer fish and robots were set to guard the path leading to the city's entrance.
Avram travelled through the mountains of Mashu on his way to Kish and encountered the robots, which he named zuqaqip, and learned from them of Utnapishtim's conflict with Qataka. He later returned there with Ace and Gilgamesh to seek Utnapishtim's help in their conflict with Qataka, who had survived their battle. (PROSE: Timewyrm: Genesys [+]John Peel, Virgin New Adventures (Virgin Books, 1991).)