thumb|Area map of the sanctuary. The coast of California is to the upper right, and the administrative centre, located on the granite outcropping, adjacent to it. The actual sanctuary is further left, and colored darker blue.
Cordell Bank National Marine Sanctuary is a marine sanctuary located off the coast of California. It protects an area of of marine wildlife. The administrative center of the sanctuary is on an offshore granite outcrop by , located on the continental shelf off of California. The outcrop is, at its closest (Point Reyes), from the sanctuary itself.
Cordell Bank is one of the United States' 13 National Marine Sanctuaries that protect and preserve ocean ecosystems in the U.S. Cordell Bank is a seamount approximately northwest of San Francisco where the ocean bottom rises to within of the surface. The seamount was discovered in 1853 by the United States Coast Survey, and named for Edward Cordell, who surveyed the area more thoroughly in 1869. It was extensively explored and described during 1978–86 by Robert Schmieder, who published a monograph about it [Schmieder, 1991]. It has been protected as a sanctuary since May 24, 1989. The protected area encompasses 526 square miles (1347 km<sup>2</sup>) of ocean.
The unique blend of ocean conditions and undersea topography creates a rich and diverse underwater ecosystem. A subsurface island rises from soft sediments covering the continental shelf. The upper pinnacles reach to within of the surface, and the average depth is . The sanctuary serves as a breeding ground for migratory marine mammals, birds, and fish. The prevailing California Current flows southward along the coast, causing an upwelling of nutrient-rich water that provided the foundation for the area's marine ecosystem.
Between 20,000 and 15,000 years ago the sea level in the area was below the current level, leaving most of Cordell Bank exposed and making it a true island. Today the bank rises out of soft sediment, deposited on the bank more recently by coastal erosion. Within just of Cordell Bank, the continental shelf drops to over deep.
In the later half of the 1800s there was a strong incentive to survey the coast of California so as to promote maritime safety. Cordell Bank was discovered in 1853 by George Davidson of the United States Coast Survey during a mapping expedition on California's north coast.
In 1869 Edward Cordell (the reserve's namesake) was sent to collect additional information on a "shoal west of Point Reyes". He found the area by following the numerous birds and marine mammals. To measure the depth, Cordell lowered a lead weight into the water until it hit the bottom and then measured the length of the line on its return to the surface.
In 2012, the National Oceanic and Atmospheric Administration proposed an expansion of the borders of the sanctuary, along with expansion of the Gulf of the Farallones National Marine Sanctuary, to include an additional 2,700 square miles, reaching to Point Arena.
Expansion was passed March 2015.
Biology
thumb|left|A school class spots a [[gray whale in the sanctuary during a field trip for the Every Kid in a Park program in 2016.]]
thumb|A [[Pacific white-sided dolphin flips out of the water in the sanctuary.]]
thumb|left|A [[tufted puffin swims in the sanctuary.]]
thumb|Rosy rockfish ([[Sebastes rosaceus) and strawberry anemones (Corynactis californica) at a depth of in the sanctuary.]]
thumb|A research [[Scuba diving|diver swims past a colony of strawberry anemones (Corynactis californica) and orange hydroids (Garveia annulata ) in the sanctuary.]]
Twenty-six species of marine mammals, including whales, dolphins, seals, and sea lions, are known to frequent the waters of the sanctuary. In addition, Cordell Bank is one of the most important feeding grounds in the world for the endangered blue and humpback whales; these species travel all the way from their breeding grounds in coastal Mexico and Central America to feed on the krill that aggregate near the bank. Another unique species is the Pacific white-sided dolphin (Sagmatias obliquidens), which can be seen in large numbers. Other visitors include California sea lions (Zalophus californianus), northern elephant seals <!--the source does not specify Northern or Southern, however California is out of the range of the southern varient--> (Mirounga angustirostris), northern fur seals (Callorhinus ursinus), and Steller sea lions (Eumetopias jubatus), all of which are attracted to the abundance in krill, squid, and juvenile fish. Leatherback sea turtles also inhabit sanctuary waters.
Cordell Bank is also a major foraging ground for passing seabirds. Known as the "Albatross capital of the world," five of the 14 major species of albatross have been documented there. The two most common are the black-footed albatross (Phoebastria nigripes) and sooty shearwater (Puffinus griseus). It is also one of the few places to see a short-tailed albatross (Phoebastria albatrus), which is extremely rare; the species was thought to have gone extinct after World War II. Currently the world population hovers at around 1000 individuals.
During the late summer and fall seasons, the coastal winds that stirred up the deeper waters die down, and the northward-flowing Davidson Current prevails, bringing warm but nutrient-poor water from the south.