"And over here is a map. As expected, there seems to be quite a large basement floor.However, only a portion of it is depicted here. Unless we can obtain another map, we won't know for certain."
Basement Blueprint (銀行地下の設計図, Ginkō chika no sekkeizu)? is a key item.
The Basement Blueprint automatically fills the map of the lower floors to the vaults in Kaneshiro's Palace.
