Tiny single-celled creatures, many of them previously unknown to science, have been found at the deepest point in the world's oceans, almost 11km down.

The soft-walled foraminifera, a form of plankton, were recovered by the Japanese remote submersible Kaiko.

Yuko Todo and colleagues report their discovery in Science magazine.

They say the organisms have become adapted to the crushing pressures that exist in a location of the Marianas Trench known as Challenger Deep.

This hole in the ocean floor is totally dark and the immense column of water above pushes down with a force that is over a thousand times greater than that at the surface - about 110,000 kilopascals.

Foraminifera are thought to be the most abundant form of life in the seas after bacteria.

They typically have shells, but these organisms are soft because there is insufficient calcium carbonate at such depth to build hard parts.

Kaiko pulled the foraminifera out of the top centimetre of sediment at Challenger Deep, 10,896m (35,748ft) below the surface.

The Marianas Trench forms part of the subduction zone where the west Pacific oceanic floor is being pulled under the Philippine tectonic plate.

The team says the deepest trenches of the western Pacific were formed about six to nine million years ago.

They write in Science: "The lineage to which the new soft-walled foraminifera belong includes the only species to have invaded fresh water and land, and analysis of the new organisms' DNA suggests they represent a primitive form of organism dating back to Precambrian times from which more complex multi-chambered organisms evolved."

Similar, though not identical, groups have been found in other, slightly shallower, ocean trenches, they add.

The foraminifera probably ingest particles of organic matter that rain down from higher up in the water column or materials that are dissolved in the seawater.

Challenger Deep was discovered in 1951 by the Royal Navy ship Challenger 2 - hence the name.

Kaiko was lost on a mission to the Nankai Trough in 2003. There is currently no remotely operated vehicle in service that can reach the bottom of the Marianas Trench.

The Science team comprises members from Shizuoka University; the Japan Agency for Marine-Earth Science and Technology; Nagasaki University; and the Southampton Oceanography Centre.