Chuuk Lagoon, as Truk Lagoon has been known since 1990, is a great natural harbour ringed by a barrier reef some 140 miles in circumference and 40-50 miles in diameter. It is one of the many lagoons and atolls that make up Chuuk State.
Rising up from the deep blue oceanic depths of the western Pacific, Chuuk State is one of the four Federated States of Micronesia – along with Yap, Pohnpei, and Kosrae – which together comprise some 607 islands scattered over almost 1,700 miles just north of the equator to the north-east of New Guinea.
During the 1920s and ’30s, Japan had developed Truk in secret as a powerful naval and air base. By the start of WWII in 1939, Truk had become the Imperial Japanese Navy’s Fourth Fleet Base, and from 1942 to 1944 it was the Japanese Combined Fleet’s main forward naval base in the Pacific.
A significant portion of the Japanese Fleet was based there – and it was a focal point for supplying and resourcing Japan’s ‘Greater Asia Prosperity Sphere’ islands and territories.
Truk had been fortified against an anticipated US amphibious invasion – with coastal-defence and anti-aircraft gun positions, and a military infrastructure of roads, trenches, bunkers, caves, five airstrips, seaplane bases, a torpedo-boat station, submarine-repair centres, a communications centre, and a radar station.
By early 1944, the Japanese garrison comprised almost 28,000 Imperial Japanese Navy (IJN) personnel and almost 17,000 Imperial Japanese Army (IJA) personnel.
The US operation
On the back foot, after the success of the Allied campaign in the Gilbert and Marshall Islands (Operations Galvanic and Flintlock) between November 1943 and early 1944, much of the Japanese Combined Fleet had retreated and gathered in their perceived stronghold of Truk Lagoon – amid frantic efforts to strengthen Truk’s defences.
But even by this stage of the war, the Allies had no clear idea of the scale of what lay in the Lagoon – it had been off limits to foreigners for decades and shrouded in great secrecy.
When the first long-range reconnaissance overflight by US Navy Liberator aircraft flying from the Solomon Islands took place on 4 February 1944, the sight that met American aviators’ eyes was astonishing.
Large elements of the Imperial Japanese Navy lay below – battleships, cruisers, submarines, escort vessels, and aircraft-carriers, along with a large number of vulnerable naval auxiliaries, merchant supply-ships, and tenders.
Allied military planners immediately started preparing a fast carrier raid by Task Force 58 – codenamed Operation Hailstone.
Knowing that the secret of Truk was out, the now vulnerable heavy IJN warships, such as Yamato and Musashi, and the valuable carriers immediately left the Lagoon. Some steamed to the relative safety of their great Pacific base at Palau, more than 1,000 miles to the west, while others headed for Singapore or Japan.
Many smaller escort destroyers, submarine chasers, and patrol boats, however, had to stay behind in Truk, while some 40 naval auxiliaries busily off-loaded their war supplies before they too could depart.
The holds and decks of these transport ships were packed with tanks, beach mines, land artillery, vehicles, aircraft, spare parts, and huge cargoes of shells for the large coastal-defence gun batteries ashore and for warships – together with huge quantities of small-arms ammunition.
The US fighter attack
Three Task Force 58 Task Groups (TG 58.1, TG 58.2, and TG 58.3) – a combined force of nine fast aircraft-carriers, screened by battleships, cruisers, destroyers, and submarines – would hit Truk with some 500 combat aircraft.
Task Group 58.4, centred around three more fast carriers and fielding 150 additional aircraft, would soften up Eniwetok Atoll in advance of a planned amphibious assault.
Knowing Japanese search patterns from intercepted radio-traffic, Task Groups 58.1, 58.2, and 58.3 successfully approached Truk undetected, and took up a holding position some 90 miles offshore late on 16 February 1944.
In the early hours of the following day, 17 February 1944, Operation Hailstone began. In darkness, 74 Grumman F6F Hellcat fighters were launched for an initial dawn sweep of Truk intended to destroy Japanese air power and suppress AA batteries.
The Hellcat fighter sweep was so swift and unexpected that, showing uncanny parallels with the Japanese attack on Pearl Harbor, the Japanese were caught by surprise and many aircraft were shot up on the ground. Others scrambled to get airborne, but were shot down as they lumbered into the air. (Today, Japanese aircraft lie all around the Lagoon – some in shallow water just hundreds of yards from the end of their airstrips.)
Those that did get airborne engaged the Hellcats in what became one of the largest aerial battles of WWII, with hundreds of fighters involved in a dogfight above the Lagoon. The fast, well-protected, and powerful Hellcats quickly triumphed over the now older and vulnerable Zeros – the skies were soon American.
The US bomber attack
With US air superiority established, throughout 17 February (code name DOG-DAY-MINUS-ONE) and into the following day, 18 February (DOG-DAY), Task Force 58 carriers launched wave after wave of Douglas SBD Dauntless and Curtiss SB2C Helldiver dive-bombers, and Grumman TBF Avenger torpedo-bombers, escorted by Hellcat fighters, to attack the now vulnerable shipping and land fortifications.
They met limited anti-aircraft fire from the lightly armed merchant ships below and from the island’s land-defences. It was a one-sided battle: on DOG-DAY-MINUS-ONE alone, 12 large Japanese ships were sunk, with many more damaged, and 72 Japanese aircraft were destroyed, with another 225 damaged.
Down below on the water, the scale of human loss was staggering. On the large 492ft-long, 10,437-tonne passenger and cargo liner Aikoku Maru, now a naval auxiliary transport vessel, 730 Japanese troops were housed in her cabins and in makeshift billets in her aft holds. Her forward holds were ominously packed with a cargo of mines, munitions, bombs, and other high explosives, along with the magazine for her defensive guns.
At about 08.30, a Mark 13 aerial torpedo from a Grumman Avenger hit her in No.1 hold in her forward section, where her deadly cargo of munitions was housed. Moments later, there was a catastrophic secondary explosion that scattered debris and parts of the ship all around the epicentre of the blast.
The forward section of the ship, from just in front of the smokestack to the bow, was almost vapourised, its pieces being spread out all over the Lagoon. A depression was blasted in the sandy seabed immediately beneath where the bow had been.
The 730 troops in her aft section were all killed in an instant, along with her crew. The remaining aft section of the ship sank quickly, taking a total of 945 troops and crew to the bottom. There was only one survivor.
Before dawn the following day, DOG-DAY, 18 February 1944, a 57-strong Hellcat initial fighter sweep crossed the lagoon, before a second day of dive- and torpedo-bomber strikes punished the beleaguered shipping in the lagoon – meeting only light AA fire.
A further 27 Japanese ships were sunk or badly damaged, and the tally of aircraft destroyed by the two-day raid rose to 250-275.
One of the few IJN naval warships sunk during Operation Hailstone was the Kamikaze-class destroyer Oite. On 15 February 1944, Oite had left Truk with submarine chaser No.28, escorting the light cruiser Agano to Japan via Saipan.
The US submarine Skate detected the naval force approximately 160 nautical miles north-west of Truk, and at sundown fired four torpedoes at Agano from a distance of 2,400 yards – scoring three hits out of the four.
She caught fire and started to sink slowly. Oite searched for the submarine fruitlessly. Skate was able to escape undetected.
Oite thereafter stayed with the stricken Agano throughout the night, receiving the transfer of Agano’s fuel and her officers and men. Oite was then ordered to return to Truk with the survivors – ignorant of the Operation Hailstone raid.
During the fighter sweep early on the second day of the raids, Hellcats from Bunker Hill and Monterey spotted Oite entering the Lagoon through North Pass. They attacked and strafed her, killing the captain in his bridge and causing fires to break out aft of the smokestack. Captain Matsuda Takatomo of the Agano assumed command of the ship.
Five Bunker Hill torpedo-bombers then joined the attack. The new skipper of Oite threw his charge about in a desperate attempt to avoid the bombs and torpedoes of the US aircraft. But, while making a high-speed evasive turn to starboard, Oite was hit by a single torpedo aft of the bridge.
The effect of the torpedo on such a relatively small, lightly protected ship was catastrophic. Travelling at full speed, she broke her back, the bow section slewing round to starboard as the aft section, her engines still turning, drove onwards. Both sections sank almost immediately.
Of the crew of the Oite, 172 were killed, while 522 survivors from the Agano – most of whom were trapped below Oite’s decks as she went down – also died. there were only some 20 survivors.
A Japanese disaster
In all, during the two days of Operation Hailstone, a total of 45 Japanese ships were sunk – more than 220,000 tons of shipping in total, a two-day record for the entire war.
As mentioned above, 250-275 Japanese aircraft had been destroyed or damaged, while 90% of the atoll’s fuel supply was set on fire, and all the airfields and other installations damaged. Truk had been completely neutralised without the need for an amphibious assault that would potentially have been costly in lives – as had been demonstrated just months before at Tarawa.
The Japanese had moored their ships in water that was deeper than their salvage diving capability. Thus the ships, along with their priceless and desperately needed cargoes, were lost to the war effort and left to lie untouched on the bottom. Today, there are still some 40 important Japanese shipwrecks in the Lagoon – along with countless aircraft wrecks.
In 1969, the legendary French oceanographer Jacques Cousteau mounted an expedition to Truk Lagoon. Many of the ship’s locations and identities were unknown at this time, but the resulting mesmeric and haunting television documentary Lagoon of Lost Ships was an instant hit around the world.
Such was the power of Cousteau’s name and the potency of the images he broadcast that the diving world’s attention focused on the Truk Lagoon wrecks. Today, Truk Lagoon holds the world’s greatest collection of diveable intact WWII Japanese shipwrecks – among them the tankers Hoyo Maru (8,691 tonnes) and Fujisan Maru (9,527 tonnes), and the oiler Shinkoku Maru (10,020 tonnes).
Exploring the wrecks
One of the most-famous wrecks in the Lagoon is the 385ft-long, 5,831-tonne IJA transport vessel San Francisco Maru. She had arrived at Truk in convoy on 5 February 1944 – and anchored in the Fourth Fleet Anchorage, south east of Dublon Island, with a mixed cargo of military hardware, stores, and munitions, all destined for the fortifications of Truk.
On her foredeck, she carried army trucks and bulldozers, along with Type 95 Mitsubishi Ha-Go¯ light tanks, that would have been deadly against lightly armed US amphibious-landing troops. Her holds were stacked with staff cars, fuel tankers, anti-invasion beach mines, fuel drums, crates of ammunition, aircraft bombs, aircraft engines, Long Lance torpedoes, and ordnance.
During the attack, she was hit by a number of 500lb bombs and set on fire amidships. The damage was catastrophic, and she went down by the stern.
Today, the wreck of the San Francisco Maru, still filled with her wartime cargo, rests upright in 60-63 metres of water, with her main deck at about 50 metres. At her very bow, a defensive gun – high above the deck on a circular platform – points out to port.
Just aft, Hold No.1 is stacked with hundreds of hemispherical beach mines, each with two grab handles welded on top – with boxes of detonators alongside. These anti-invasion mines had been destined to be buried in hundreds on beaches that the Allies were expected to assault.
In the Tween Deck of Hold No.2 are two tanker trucks and a staff car, while on the deck level below, 50lb aerial bombs are stacked, with their tailfins upmost, alongside more artillery shells and radial aircraft engines.
Just in front of the bridge superstructure on the starboard side of the main deck are two Type 95 Ha-Go¯ light tanks. On the port side of the deck sits another Type 95 tank, resting partly on the gunwale. A large steamroller lies on the seabed nearby, on the port side.
The aft holds contain lorries, artillery shells in boxes, more hemispherical beach mines, detonators, bombs, small-arms ammunition, and 55-gallon fuel drums. Hold No.5 contains dozens of 30ft Long Lance torpedo bodies and engines, along with a large number of 55-gallon fuel drums.
The 461ft-long, 9,627-tonne Rio de Janeiro Maru was a substantial eight-deck passenger-cargo liner, built in 1929 and able to accommodate 1,140 peacetime passengers. On 3 February 1944, now a submarine tender attached to the Combined Fleet and filled with depth charges, coastal-defence guns, and general supplies, she set off from Yokosuka in Japan for Truk, escorted by the Mutsuki-class destroyer Yu¯zuki. She arrived in Truk on 11 February 1944, just six days before Operation Hailstone.
Following the dawn Hellcat fighter sweep on 17 February 1944, she was struck by several 1,000lb bombs during the first group strikes of the day, causing severe damage, starting intense fires aboard, and eventually sending her to the bottom of the Lagoon.
At 461ft in length, the Rio de Janeiro Maru is one of the largest wrecks in the Lagoon, and still has her name in large non-ferrous letters in both Roman script and kanji (Japanese characters), almost 2ft high, on her bow and around her stern.
In her holds are circular artillery base supports, some 15ft in diameter, destined for installation on the land, along with recoil springs and a number of large-bore artillery barrels. The Engine Room still holds her Mitsubishi Sulzer diesel engines, its walls – now horizontal – covered with switching panels, gauges, and two telegraph repeaters. •
Rod Macdonald is an internationally renowned shipwreck explorer, undersea adventurer, and bestselling diving author, with 11 books to his name, including Dive Truk Lagoon – the Japanese WWII Pacific Shipwrecks and Dive Palau – the Shipwrecks.
His dedication to diving shipwrecks has led him to be the first to explore and survey many lost wrecks, and his beautifully illustrated guides to world-famous dive locations such as Scapa Flow, Truk Lagoon, and Palau are accepted internationally as definitive.
Rod is a Patron of the Great Britain and Ireland Chapter of the Explorers Club of New York, and, when not writing or exploring shipwrecks, he is a big-boat yachtsman and a lifeboat Search and Rescue (SAR) instructor.