Nickel (Ni) is a critical alloying metal in the production of steel armor. Japan had some domestic production, supplemented with some from Korea, but they were totally inadequate for wartime needs. Part of Japan's war plans were to seize significant nickel production areas in Celebes Island, DEI (Soroaka mines), the Badwin mines in Burma and in New Caledonia (at the time the richest known nickel deposits in the world). Japan also needed nickel to use as filaments in vacuum tubes. They resorted to melting down nickel coins they acquired in Hong Kong. They attempted to use copper in place of nickel for armor production but the resulting armor was not as good.