Bunker and workshop for Hephaestus' cabin
Bunker 9 is a hidden bunker that Festus led Leo Valdez to. It is an aircraft hanger sized workshop filled with tools and weapons, schematics, a map of the camp, and various machine design plans. It is at least 200 years old, used in the Demigod Civil War, and on a few occasions since. Leo was able to open the bunker using his fire abilities, so it is not known how other demigods gained access.
Leo finds his Magical Toolbelt inside here. It was revealed that Festus was created in this Bunker. The Argo II's blueprints were found here. The Bunker can be only opened and closed by a blast of fire, as shown when Leo and Festus each opened it by setting it on fire, probably as a safety measure against enemies.