As WW II approached, portable storage units were replaced with extensive underground rooms and tunnels for ammunition storage at many locations on O‘ahu. One worker commented that the Engineers had built so many tunnels, if placed end to end the entrance would be at Koko Head, the exit at Moanalua.
At the onset of World War II, the Army was importing ammunition in huge quantities, requiring construction of ammunition storage facilities. Small facilities were built above ground, but the bulk of the ammunition was stored in massive underground storage facilities. Tunnel complexes were built, including Schofield Barracks, Wheeler Field, Fort Shafter and Fort Ruger.