Some of Earth's most impressive topological features are hidden under the sea, including mountains higher and valleys deeper than any that exist on land. The largest mountains in the world, Mauna Loa and Mauna Kea, rise from the Hawaiian Trench, some 5,500 meters (18,000 feet) below sea level, but that's almost a plateau compared to some of the deep ocean trenches. The movement of Earth's plates -- the layers of rock that cover the planet's hot, flowing mantle -- produces these trenches, which can be almost 11 kilometers (7 miles) deep. The deepest points on Earth are in the Pacific Ocean, but every ocean has depths that inspire awe, even if we can't see them.
Until 1970, scientists believed the Philippine Trench, which stretches southwest from Luzon to the island of Halmahera in Indonesia, was the deepest point on the planet. It's the result of a collision between the Eurasian plate, which is one of the Earths' seven major tectonic plates, and the smaller Philippine plate. As the larger plate slides over it it, the smaller plate, which is denser, sinks into the Earth's mantle, where it melts. The process, called subduction, forms the V-shape of the trench. At its deepest point, the Philippine Trench is 10,540 meters (34,580 feet) below sea level.
The Tonga Trench stretches from New Zealand's North Island northeast to the island of Tonga, a distance of 2,500 kilometers (1,550 miles). Formed by subduction of the Pacific plate by the Tonga plate, it boasts the second-deepest point on the planet -- the Horizon Deep -- which is 10,882 meters (35,702 feet) below sea level. Researchers have discovered that plate movement in the Tonga causes large volcanoes to slip into the abyss, as well as in the Japan trench to the north and the Mariana trench to the south. Such cataclysms can cause massive earthquakes and tsunamis, such as the one that struck Japan in 2011. In 2013, Japanese researchers descended into the Horizon Deep and brought back a 24-centimeter (9.5-inch) prawn-like amphipod -- Alicella gigantea -- from a depth of 6,250 meters (20,500 feet). Devoid of pigmentation, the creature survives in total darkness in pressures close to 1,000 atmospheres.
Just southeast of the southern tip of South America, the British territories of South Georgia and the South Sandwich Islands provide a home for penguins and a few British administrative personnel. Just to the east, the ocean floor dips into the South Sandwich Trench, the second-deepest trench in the Atlantic Ocean. At its lowest point, this trench is 8,428 meters (27,651 feet) below sea level. Subduction of the South Atlantic plate by the Scotia plate formed this trench, as well as the archipelago of islands, also known as the Scotia Arc, that extends to the tip of Antarctica.
The deepest part of the Atlantic Ocean lies just north of the island of Puerto Rico, where the North American and Caribbean plates slide past each other. Subduction of the larger North American plate by the Caribbean plate has created a trench that is 8,605 meters (28,232 feet) deep. The interaction produces earthquakes in the region -- as such plate interactions do worldwide -- but a recent study has shown that there is a greater hazard. As the plates collide, the lighter Caribbean plate cracks and ruptures, while giant landslides occur on the down-going North American plate. Both phenomena, which are also common in the deeper Pacific trenches, are capable of producing devastating tsunamis.
A mountain range separates the ocean floor into the Eurasian and Amerasian Basins under the Arctic Sea, and the former descends to a depth of 4,400 meters (14,435 feet) in the Barents Abyssal Plain. This depth is part of the Fram Basin, which lies directly under the geographic North Pole. Unlike ocean trenches, the Fram Basin isn't V-shaped, but vast and flat, much like the floor of a desert on dry land. Scientists haven't completely mapped the Arctic Ocean floor, but they know that, beneath the Fram Strait between Greenland and Svalbard, it descends to a depth of 5,607 meters (18,395 feet) in the Molloy Deep.
Long ago, Australia used to be part of Antarctica, but as they drifted apart, fracture zones were created in the Earth's crust. One of these fractures produced the Diamantina Trench, just off the southwestern tip of Australia. With a maximum depth of 8,047 meters (26,401 feet), it's the deepest part of the Indian Ocean, and it's the eleventh deepest trench in the world. If the base of Mount Everest was at the same depth, its peak would form an island with a maximum elevation of about 900 meters (3,000 feet).
The Mariana Trench is the deepest of all ocean trenches. Formed by the same plates that created the Philippine Trench, the Mariana Trench is just northeast of that slightly shallower one, to the east of the Mariana Island chain and just south of Japan. The deepest part, known as the Challenger Deep, is 10,911 meters (35,797 feet) below sea level. Hollywood director James Cameron made a solo descent to the bottom of the trench in 2012, but he wasn't the first person to visit. Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Don Walsh touched down in the bathyscaphe Trieste in 1960. Despite the 200,000 tons of water pressure at that depth, Piccard managed to catch sight of a foot-long sole scouring the ocean floor for food.
- Monterey Bay Aquarium Research Institute: Hawaiian Flexural Arch
- Woods Hole Oceanographic: Trenches
- Stuff.co.nz: Frightening Bugs at Extreme Depth
- Oceana: South Sandwich Trench
- United States Geological Service: Mapping of the Puerto Rico Trench, the Deepest Part of the Atlantic, is Nearing Completion
- The Encyclopedia of Earth: Arctic Ocean
- International Ocean Discovery Program: Bathymetry of Molloy Deep: Fram Strait between Svalbard and Greenland
- Channel4.com: MH370: What Lies Beneath the Southern Indian Ocean?
- Deep Sea Challenge: 1960 Dive
- Steve Horstmeyer: Geo 130 - Oceanography; Plates and Trenches