than on passive continental margins. The walls are generally very steep and can be near vertical. The walls are subject to erosion by bioerosion, or slumping. There are an estimated 9,477 submarine canyons on Earth, covering about 11% of the continental slope. Examples Avilés Canyon, off the coast of Asturias, Spain, reaches depth of about 4,700 m. Amazon Canyon, extending from the Amazon River Baltimore and Wilmington Canyons, East Coast of Maryland and Delaware States Bering Canyon, in the Bering Sea Congo Canyon, the largest river canyon, extending from the Congo River, is long, and deep. Hatteras Canyon, off the coast of North Carolina Hudson Canyon, extending from the Hudson River Ganges Canyon, extending from the Ganges Indus Canyon, extending from the Indus River Kaikōura Canyon, extending offshore from the Kaikoura Peninsula, New Zealand La Jolla and Scripps Canyon, off the coast of La Jolla, Southern California Mona Canyon, off the coast of western Puerto Rico Monterey Canyon, off the coast of central California Nazaré Canyon, off the coast of Portugal, reaches abyssal depths of 5,000 m Perth Canyon, off the coast of Perth, Western Australia, reaches a depth of 4,000 m and contains the world's largest plunge pool Pribilof Canyon, in the Bering Sea Whittard Canyon, Atlantic Ocean off southwest Ireland Zhemchug Canyon the deepest and joint-widest submarine canyon in the world, in the Bering Sea Formation Different mechanisms have been proposed for the formation of submarine canyons. Their primary causes have been subject to debate since the early 1930s. An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about 125 meters below present sea level, and rivers flowed to the edge of the continental shelf. However, while many (but not all) canyons are found offshore from major rivers, subaerial river erosion cannot have been active to the water depths as great as 3000 meters where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths. The major mechanism of canyon erosion is thought to be turbidity currents and underwater landslides. Turbidity currents are dense, sediment-laden currents which flow downslope when an unstable mass of sediment that has been rapidly deposited on the upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from "muddy water" to massive