AS one enters and exits Manila Bay, there is no more arresting sight than a ruined concrete fort jutting out of the sea with four big guns pointing seaward.

Passengers of ships passing near the island usually gawk at the ruined fort, wondering what it is. In all my trips to Mindanao for the past three years, I have heard no one identifying the fort correctly.

The ruined concrete fortification is Fort Drum, formerly El Fraile Island, a concrete battleship, the only unsinkable battleship in the world, and one of the greatest military fortifications of all time.

Fort Drum is a historic island and military fortification. It deserves to be promoted as a tourist spot, and its story is worth retelling.

Philippine defense became the United States' responsibility when it annexed the country in 1898. As part of its defense plan for its new colony against future invaders, the United States fortified four islands at the mouth of Manila Bay beginning 1909 up to 1913.

The four islands were Corregidor, Caballo, Carabao and El Fraile, which became Fort MIlls, Fort Hughes, Fort Frank, and Fort Drum, respectively.

According to War Plan Orange, the forts were to deny enemy warships from entering Manila Bay and to provide assistance in Bataan where Filipino and American soldiers were to fight a delaying action for six months.

Of the forts, Corregidor was the biggest and most important; however, El Fraile or Fort Drum, the smallest, was the most unique. It was shaped like a battleship complete with a forecastle. No wonder Fort Drum was also called USS Drum-- because passengers of passing ocean liners often mistook it for a ship, albeit a strange one.

To build Fort Drum, the US Army Corps of Engineers cut El Fraile, a small rocky island to the mean water line, and, using the rock as foundation, erected a concrete fortification shaped like a battleship. The ''battleship'' was 240 feet long, 160 feet wide, and 40 feet above the water line. The walls were 30-40 feet thick and the deck 20 feet deep. Inside, there were four levels connected by an axial tunnel running through the island.

Fort Drum bristled with 11 guns: Battery Wilson, a rotating turret with two 14-inch guns that can sink any known warship within 22,500 yards; Battery Marshall, the rotating turret at the front, also with two 14-inchers; Battery Roberts, a casemated battery with four 6-inch guns for minefield defense; and a battery of three 3-inch guns two of which were anti-aircraft guns.

In addition, Fort Drum had two 8-foot searchlights for night fighting. A garrison of 200 men were stationed at the fort.

It took 11 years to construct Fort Drum, from 1909 to 1919. When it was completed, it was considered impregnable to all known armaments then, and impregnable it turned out to be.

War came to the Philippines with the bombing of Pearl Harbor in December 1941. Fort Drum received its baptism of fire when Homma's air force bombed Corregidor and its sister islands on Dec. 29, 1941, and again on Jan. 2-6, 1942.

Fort Drum was hardly nicked. Corregidor was heavily damaged, but not its guns, the anti-aircraft taking an appreciable toll on Japanese planes. The raids were costly and did not impair the fighting capabilities of the fortified islands. Thus, the Japanese stopped the bombing until later.

Beginning Jan. 25, when the Usaffe was ensconced in Bataan, the Japanese began emplacing artillery pieces in Ternate, Cavite, to shell the island forts. The battery, commanded by Maj. Toshinori Kondo, commenced shelling on Feb. 5 with four 105-mm guns and two 150-mm howitzers. The prime target on the first day was Fort Drum, which was hit 100 times without effect.

From then on, Kondo's fire on the four islands became regular. It intensified in mid-February with the addition of two more 150-mm howitzers.

The defenders responded with their big guns, but they were handicapped by lack of forward observers to give them the exact locations of Kondo's guns. Only after Maj. Jess Villamor successfully took aerial photos of the Japanese batterries did they score direct hits.

By late February, the fire from Kondo's guns had diminished.

But while Kondo's fire was slackening, the Japanese were emplacing 10 240-mm howitzers in the Pico de Loro hills in Calumpang, Cavite, close to Fort Frank. This new artillery detachment under Maj. Masayoshi Hayakawa started unleashing deadly shells against the four islands on March 15.

Hayakawa's shells, the deadliest in the Japanese arsenal, damaged most of Fort Frank's guns. Fort Drum's two searchlights and two anti-aircraft guns were destroyed, but not its 14-inch batterries even though many shells landed on the top, sides and face of the turrets.

Fortunately, Hayakawa's monsters were pulled out to Bataan on March 22 to join in the final assault of the peninsula. By then, Fort Drum was pock-marked with hits from Japanese shells which chipped at least four inches of concrete.

After Bataan surrendered on April 9, 1942, the Japanese began preparations for taking Corregidor and its sister islands.

Starting April 11, the Japanese started shelling Corregidor, Fort Hughes, Fort Frank, and Fort Drum with 110 guns ranging from 75 mm to 240 mm. The guns of Corregidor, Fort Hughes, and Fort Frank countered as best they could, but it was an unequal artillery duel.

The Japanese not only had more guns but also had observers on the ground and in the air with sensitive instrument for range-finding the islands' batteries. Moreover, an average of 50 Japanese bombers had been bombing the islands since March 24. The defenders could lob only a few salvoes before being plastered with Japanese shells. But the guns of Fort Drum were never silenced, affording the defenders much needed protection.

Japanese shellings and bombings intensified on April 29, Emperor Hirohito's birthday. To get rid of the pesky guns of Fort Drum, the Japanese subjected the fort to a glide-bombing attack which effected only a minor misalignment of Battery Marshall. The intense bombing-shelling continued for the next four days.

By May 5, Corregidor's guns had been silenced, except for one 12-inch 1898 mortar of Battery Way and a few roving 155- and 75-mm guns which had not disclosed their positions.

On the night of May 5, the Japanese launched their 2-battallion Corregidor invasion force. The gallant defenders destroyed two-thirds of the invaders, but the remainder made a successful beachhead with armor and three tanks.

The defenders were outflanked, and reinforcements were immediately shelled by Japanese guns to keep them from containing the invaders. General Wainwright had no choice but to surrender Corregidor on May 6.

Through all these, Fort Drum's guns continued to blaze until minutes before the surrender.

The Americans returned on Oct. 20, 1944, and started the liberation of the Philippines. By Feb. 3, 1945, a flying column had reached Manila and a month-long battle liberating Manila from the Japanese ensued.

While the battle of Manila was raging, the Americans started clearing the fortified islands of Japanese to open Manila Bay for shipping. Fort Drum was the last to be liberated.

To liberate Fort Drum, which was impregnable to gunfire, the Americans devised special tactics. On Friday, April 13, a Landing Ship Medium (LSM) pulled up alongside Fort Drum and discharged two platoons of soldiers by means of a specially built ramp on top of the LSM. One platoon consisted of crack snipers to cover every opening where Japanese soldiers may appear. The other platoon comprised engineers assigned to plant demolition charges.

When the charges were in place, a Landing Craft Mechanized (LCM) that had also sidled alongside the fort poured 3,000 gallons of oil into one of the vents while explosives were dumped into another vent. The fuses were lit, and the LCM and LSM moved to a safe distance. The charges were detonated, resulting in a series of explosions that hurled Fort Drum's one-ton, one-meter diameter manhole cover 50 meters straight up into the air.

It was not until April 18 that the Americans could enter the fort. They discovered 65 charred bodies.