The Factory 1010 Station Gate bordered by the Hydro-Wall in Battle 31
The Hydro-Wall is a defensive wall surrounding the outer boundary of the Scrapyard, revealed in a map in volume 1 of Gunnm Works to form a massive circle around the city. It is a fountain of super viscous liquid that is projected 20 meters into the air by a high pressure electromagnetic pump. The only breaks in the Hydro-Wall are the 11 Station Gates which control access to the Scrapyard.
The Hydro-Wall was first described after Alita had been recruited to become a TUNED agent and the story moved forward ten years. When Den charged the Scrapyard in ES 590, he smashed through the Factory 11 Station Gate, leaving the Hydro-Wall bordering it intact.