Bunker 9 is a hidden bunker in the Woods that Festus led Leo Valdez to. It is an aircraft hanger sized workshop filled with tools, 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.
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, so it is not known how other demigods gained access.
Bunker 9 is presumed to have been the Hephaestus Cabin's Bunker during the war. It is said that all the ideas would take at least a century to build.
( http://camphalfblood.wikia.com/wiki/Bunker_9 )
