North Korea developed a theory of underground warfare. Manpower, warehouses and small calibre guns were completely housed underground making them less vulnerable to air strikes and artillery. On the surface the many false targets (bunkers, trenches and decoy entrances to the tunnel system) made it difficult to detect true targets forcing US forces to waste ammunition. Directly under the surface spacious barracks were built, allowing whole units to be quickly brought to the surface for a short time and as quickly returned to shelter underground.