An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3000 and 6000 m. Abyssal plains cover more than 50% of the Earth’s surface. The larger plains are hundreds of kilometres wide and thousands of kilometres long. The plains are largest and most common in the Atlantic Ocean, less common in the Indian Ocean, and even rarer in the Pacific, where they occur mainly as the small, flat floors of marginal seas. The plains are made up of sediment from the land that has slumped down in depressions in the seabed, smoothing out the bottom. Sediments can average one kilometre in thickness. The coarser layers of sand and gravel are interspersed with fine-grained clay and the microscopic remains of organisms that have died and fallen to the seafloor.

Abyssal Plains are underlain by oceanic crust of basalt, a dark colored volcanic rock rich in iron- and magnesium-silicate minerals. Fine-grained sediments accumulate at a millimetre to several centimetres every 1,000 years. Only a fraction of the 15 billion tons of clay, silt, sand, and gravel that is washed into the oceans each year reaches the abyssal plains. The other minor components of abyssal plain sediment include windblown dust, volcanic ash, chemical precipitates, and occasional meteorite fragments. Abyssal plains are often littered with manganese nodules containing varying amounts of iron, nickel, cobalt, and copper. The nodules form by direct precipitation of minerals from the seawater onto a bone or rock fragment.

Abyssal plains were once thought to be stable and unchanging environments. ”The abyssal plain environment is not conducive to life as we know it; it is perpetually dark and very cold, and the food supply is sparse. The Hydrostatic pressure is enough to crush a person’s body to the size of a soccer ball. Population densities are low owing to a harsh environment and scarcity of food”” Instead there is high biodiversity, “one of the major biogeographic puzzles of our time”. The abyssal seafloor can be unexpectedly dynamic. Nearly all species found in the abyss are rare. Most species have been recorded as one or two individuals from one or two sampling sites, even in large programs. One or a few species numerically dominate but can change quickly.

Industrial harvesting of manganese nodules may become a reality. The abyssal seafloor, which accounts for the largest area on the planet, may also warrant our close attention because biogeochemical cycles of the seafloor have a strong influence on the global climate and climate change.

The recent search for a Malaysian Boeing 777 that tragically disappeared, resulted in one of the most detailed surveys of the deep waters of WA ever attempted. Bathymetric survey vessels spent months scanning with multibeam sonar pulled along the sea floor by a 10km armoured cable. The 28 resolution was too coarse to find the aircraft, but making sonar maps are needed to ensure the team did not crash its deep-water vehicles into ridges. Previous satellite maps only indicated the depth of the ocean. They found a mountainous ridge that once formed the margin between two geological plates. These plates evolved and spread apart between 20 and 100 million years ago. There are extinct volcanoes, rugged ridges up to 300 metres high and trenches some 1,400 metres deep in this part of the search area. Simon Boxall from the National Oceanography Centre said “Those ‘bumps’ on the sea floor in the flat, featureless plains to the south of Broken Ridge are each bigger than Ben Nevis. “Five kilometres (3 miles) across and typically rising 1.5km (0.9 miles) from the sea floor. The terrain of the area around Broken Ridge makes the European Alps look like foothills,” he said. It has also revealed regions of harder and softer sea floor composition (sediment versus rock).