Probably the only US 12-inch mortars to see action were those in the Philippines in 1942, particularly Battery Way and Battery Geary at Fort Mills on Corregidor. These batteries totaled twelve mortars and were used in the fight against the Japanese invasion. However, as with other US pre-1940 fortifications (most of which were designed well before the airplane was invented), the open-top emplacement design left the mortars open to air and high-angle artillery attack. All but two of the batteries' mortars were knocked out by the latter, all but one of Battery Geary's by a single hit that penetrated the magazine. The mortar batteries had mostly armor-piercing ammunition which would penetrate into the ground before exploding, and therefore was less effective than high explosive ammunition against ground forces. Other 12-inch mortars were Battery Koehler on Carabao Island (Fort Frank), which was used for ineffectual counter-battery fire against the Japanese Kondo Detachment artillery on the southern shore of Manila Bay, and Battery Craighill on Caballo Island (Fort Hughes), which fired at targets generated locally and from Corregidor. Battery Craighill was manned by naval personnel from the gunboat "Mindanao". All forts were surrendered on 6 May 1942 along with Corregidor. Ten mortars remain at Batteries Way and Geary while four mortars remain at Fort Hughes as of 2014.