The Philippine Sea is a marginal sea of the western Pacific Ocean, east of the Philippines and Taiwan and south of Japan, and at an estimated 5 million km² (2 million sq mi) it is often described as the largest sea in the world. Its floor is a single tectonic plate, the Philippine Sea Plate, ringed by island arcs and some of the deepest trenches on Earth: the Philippine Trench off Mindanao plunges to 10,540 m, and the Mariana Trench along its eastern rim holds the Challenger Deep, at about 10,935 m the deepest known point of the ocean floor. Its western islands run from the Ryukyus and Taiwan to Luzon, Samar and Mindanao; its eastern rim runs from the Bonin Islands through the Marianas, including Guam and Saipan; Palau, Yap and Halmahera close the south. The warm Kuroshio Current begins off its western edge, and it lies in the western Pacific, the most active tropical-cyclone basin on Earth.
The Philippine Sea Plate and Its Sinking Edges
The Philippine Sea Plate is oceanic lithosphere bounded almost entirely by convergent edges, so it is either diving under its neighbours or being dived under. Its oldest part, the West Philippine Basin, formed in the early Oligocene and is cut by the Central Basin Fault, a band of high relief running from the north-west corner towards the Kyushu–Palau Ridge, which splits the sea's floor into western and eastern basins.
On the north side the plate slides beneath Japan along the Nankai Trough, a trough about 900 km long off Honshu whose megathrust has produced Japan's great Nankai earthquakes; it meets the Okhotsk and Amurian plates near Mount Fuji. On the north-west it dives under the Ryukyu Islands along the Ryukyu Trench, 1,398 km long and 7,460 m deep, at about 52 mm a year, a process that, together with the Nankai Trough, has built 34 volcanoes. On the east the roles reverse: the Pacific Plate dives beneath the Philippine Sea Plate along the Izu–Ogasawara Trench, which reaches 9,826 m and was first dived to its base on 13 August 2022. On the west the Philippine Sea Plate goes under the Philippine Mobile Belt, which carries most of the Philippine archipelago and eastern Taiwan.
Challenger Deep and the Mariana Trench
The International Hydrographic Organization draws the sea's eastern limit along the ridge joining Japan to the Bonin, Volcano and Mariana Islands, and it includes those islands in the sea. The Izu–Bonin–Mariana arc that forms this rim runs for more than 2,800 km south from Tokyo to beyond Guam. Its volcanoes are fed by water released from the Pacific Plate as it sinks, and that sinking crust is some of the oldest oceanic crust on Earth, up to about 170 million years old, cold and dense enough to fall steeply.
The result, on the ocean side of the Marianas, is the Mariana Trench, a crescent about 2,550 km long and 69 km wide lying roughly 200 km east of the islands. At its southern end lies the Challenger Deep, a slot-shaped depression of three basins. A 2021 study put its depth at 10,935 ± 6 m, more than 2 km farther below sea level than Mount Everest's 8,848.86 m summit rises above it. The water above presses down at about 1,086 bar, and the bottom water is between 1 and 4 °C.
HMS Challenger first sounded the trench in 1875 with a weighted rope, recording 8,184 m, and HMS Challenger II established its record depth during its 1950–1952 voyage. The US Navy bathyscaphe Trieste made the first descent in January 1960, and by July 2022 a total of 27 people had been to the bottom.
The deep does not belong to the islands it sits beside. In 2009 the United States created the Mariana Trench Marine National Monument from the parts of the trench inside the US exclusive economic zone, but the Challenger Deep itself lies in the zone of the Federated States of Micronesia. Its nearest land is Fais Island in Yap, 287 km to the south-west, with Guam 304 km to the north-east.
Emden Deep and the Philippine Trench off Mindanao
On the opposite, western side of the sea the plate sinks into the Philippine Trench. It runs about 1,320 km from central Luzon to Halmahera in Indonesia, is roughly 30 km wide and bottoms out at the Emden Deep, 10,540 m below the surface. It is young, probably less than 8 to 9 million years old, and its northern continuation, the East Luzon Trench, is offset by the Benham Rise, an extinct volcanic ridge about 250 km east of Dinapigue in Isabela province.
Some authors have described the region as seismically quiet, yet large earthquakes strike along the trench. Southern Mindanao had a magnitude 8.2 earthquake in 1924, and the 2012 Samar earthquake reached magnitude 7.6. Because the Battle off Samar was fought above it in October 1944, it also holds the two deepest known shipwrecks: the destroyer escort USS Samuel B. Roberts at 6,890 m and the destroyer USS Johnston at 6,460 m.
The Benham Rise shows how the sea's floor is also a map of claims. The Philippines filed it as part of its continental shelf with the UN Commission on the Limits of the Continental Shelf on 8 April 2009; the claim was approved in 2012, and Executive Order No. 25 renamed the feature Philippine Rise on 16 May 2017. The government has banned mining and oil exploration there; China maintains that the Philippines has no sovereign rights over the rise, despite the 2012 approval.
The Kuroshio Current from Luzon to the Bōsō Peninsula
The Kuroshio is born on the sea's western edge. The westward-flowing North Equatorial Current hits the east coast of Luzon and splits: a smaller branch turns south as the Mindanao Current, and the larger branch turns north as the Kuroshio itself, or "Black Stream", named for the deep blue of its water. It is about 100 km wide with an average surface temperature of around 24 °C.
From Luzon the Kuroshio passes east of Taiwan, enters the East China Sea through the Yonaguni Depression, follows the Okinawa Trough parallel to the Ryukyus and returns to the Pacific through the Tokara Strait. It then meanders along southern Japan before leaving the coast at the Bōsō Peninsula as the Kuroshio Extension. Its flow strengthens to about 65 million cubic metres per second south-east of Japan, and its warmth sustains Japan's coral reefs, the northernmost in the world. Europeans learned of it early: the Spanish navigator Andrés de Urdaneta is credited with discovering it in 1565, on the return voyage from Cebu to New Spain that set the route of the Manila galleons.
Typhoon Alley: Tip in 1979 and Haiyan in 2013
Nearly one third of the world's tropical cyclones form in the western Pacific, which makes it the most active basin on Earth, and the area just north-east of the Philippines is the most active place of all. Typhoons have formed in every month, but the peak runs from August to October. The Philippine Area of Responsibility sees about 19 typhoons in a typical year, mostly between July and October, and eight or nine of them make landfall. Islands facing the open sea, such as Catanduanes and Samar, take the first blow.
Two storms from this basin set records. Typhoon Tip formed near Pohnpei in Micronesia on 4 October 1979, intensified after passing Guam and on 12 October reached 305 km/h winds and a central pressure of 870 hPa, a worldwide record low for sea-level pressure. Its wind field measured 2,220 km across, making it the largest tropical cyclone on record. Typhoon Haiyan, known in the Philippines as Yolanda, crossed the sea in November 2013, passing over Kayangel in Palau as a Category 5-equivalent storm. When it made landfall at Guiuan, Eastern Samar, PAGASA estimated ten-minute sustained winds of 235 km/h with gusts of 275 km/h. It killed at least 6,300 people in the Visayas.
Okinotorishima, 1,740 km South of Tokyo
In the middle of the sea, on the Palau–Kyushu Ridge, lies Okinotorishima, also called Parece Vela, Spanish for "looks like a sail". It is a coral atoll whose reef encloses a lagoon about 4.5 km long and 1.7 km wide. At high tide its two natural rocks rise only 7.4 cm and 16 cm above the water, and their combined land area is 9.44 m². It is the southernmost point of Japan and the only Japanese territory south of the Tropic of Cancer.
Miguel López de Legazpi made the first undisputed sighting in 1565. Japan declared it Japanese territory in 1931; the United States administered it with the Bonin Islands after the Second World War and returned it in 1968. Between 1987 and 1993 Japan ringed the rocks with steel breakwaters and concrete, later covering the smaller rock with a titanium net. Japan claims an exclusive economic zone of about 400,000 km² around the atoll. China, South Korea and Taiwan reject that claim. In their reading the atoll fails the island test of the Law of the Sea convention: its Article 121(3) denies an exclusive economic zone to rocks that cannot sustain human habitation or economic life of their own.
From Magellan's Mar Filipinas to the Battle of June 1944
Ferdinand Magellan was the first European to sail the sea, in 1521, and named it Mar Filipinas while in the Mariana Islands. Four centuries later, on 19 and 20 June 1944, the US Fifth Fleet and the Imperial Japanese Navy's Mobile Fleet fought the Battle of the Philippine Sea near the Marianas, the largest carrier-to-carrier battle in history. American pilots called the air battle the "Great Marianas Turkey Shoot" for the number of Japanese aircraft shot down.
Today the sea's western margin lies inside the Coral Triangle and supports the livelihoods of an estimated 120 million people, with more than 3,212 fish species and 486 coral species recorded. For the full story of the deepest trench, read our Mariana Trench guide; the Kuroshio Current guide follows the current north to Japan, and the climate of the Philippines explains what the typhoon season means on land.