The Greenland shark, Somniosus microcephalus, also known as the sleeper shark, gurry shark, ground shark, grey shark, or by the Inuit languages name Eqalussuaq, is a large shark native to the waters of the North Atlantic Ocean around Greenland and Iceland. These sharks live farther north than any other shark species. They are closely related to the Pacific sleeper shark. This is one of the largest species of shark, of dimensions comparable to those of the great white shark. Large Greenland sharks grow to 6.4 metres (21 ft) and 1,000 kilograms (2,200 lb), and possibly up to 7.3 metres (24 ft) and more than 1,400 kilograms (3,100 lb). However, most Greenland sharks observed have been around 3–4.8 metres (9.8–15.7 ft) long and weigh up to 400 kilograms (880 lb). It rivals the Pacific sleeper shark (possibly up to 7 m or 23 ft long) as the largest species in the family Somniosidae. There are no reliable data on their life span, but fully grown Greenland sharks have been recaptured 16 years after being tagged.
The Greenland shark is an apex predator mostly eating fish, though it may also prey on marine mammals such as seals. Bite marks on dead seals at Sable Island, Nova Scotia and Hawarden suggest that this shark may be a major predator for them in the winter months. Greenland sharks have also been found with remains of polar bear and reindeer in their stomachs. The Greenland shark is also known to be a scavenger but to what extent carrion (almost certainly the origin of the reindeer) figures into the slow-moving fish's stomach contents is unknown. The shark is colonized by the parasitic copepod Ommatokoita elongata that eats the shark's corneal tissue. The shark occupies what tends to be a very deep environment seeking its preferable cold water (-0.6 to 10 °C (31 to 50 °F)) habitat. It has been observed at a depth of 2,200 m (7,200 ft) by a submersible investigating the wreck of the SS Central America. A specimen video-taped at 9,100 feet (2,770 m) off the coast of Brazil on February 11, 2012 may have been a Greenland shark, but cannot be distinguished in the video from a southern sleeper shark or Pacific sleeper shark..
As recently as 1957 it was found that the females do not deposit eggs in the bottom ooze, but retain the developing embryos within their bodies so that they are born alive after an undetermined gestation period. 10 pups per litter is normal, each measuring some 90 centimetres in length. 
Greenland sharks as food
The flesh of a Greenland shark is poisonous. This is due to the presence of the toxin trimethylamine oxide, which, upon digestion, breaks down into trimethylamine, producing effects similar to extreme drunkenness. Occasionally, sled dogs that end up eating the flesh are unable to stand up due to the neurotoxins. Similar toxic effects occur with the related Pacific sleeper shark, but not in most other shark species, whose meat is often consumed fresh.
However, it can be eaten if it is boiled in several changes of water or dried or fermented for some months to produce Kæstur Hákarl, often Hákarl for short. Traditionally this was done by burying the shark in boreal ground, exposing it to several cycles of freezing and thawing. It is considered a delicacy in Iceland and Greenland.
The Greenland shark's poisonous flesh has a high urea content, which gave rise to the Inuit legend of Skalugsuak, the first Greenland shark. The legend says that an old woman washed her hair in urine and dried it with a cloth. The cloth blew into the ocean to become Skalugsuak.
The Greenland shark occupies a minor role within Inuit cosmologies in the Canadian Eastern Arctic and Greenland. For the Igloolik Inuit, this fish lives within Sedna's urine pot (hence the urine-like smell of its flesh) and is conceived as a shaman’s helping spirit (Idrobo 2009)
When feeding on large carcasses, the shark employs a rolling motion of its jaw. The teeth of the upper jaw act as anchor while the lower jaw does the cutting. Teeth in the two halves of the lower jaw are strongly pitched in opposite directions. 
The Greenland Shark and Elasmobranch Education and Research Group (GEERG) has been studying the Greenland shark in the Saguenay Fjord and St. Lawrence Estuary since 2001. The Greenland shark has repeatedly been documented (captured or washed ashore) in the Saguenay since at least 1888. Accidental captures and strandings have also been recorded in the St. Lawrence Estuary for over a century.
Current research conducted by GEERG involves the study of the behaviour of the Greenland shark by observing it underwater using scuba and video equipment and by placing acoustic and satellite tags (telemetry) on live specimens; however, overall very little is known about this mysterious species.
There is now an argument suggesting that the Greenland shark is responsible for 'Seal Ripping' attacks on grey seals. These attacks cause a corkscrew pattern of cuts and tears which spiral around a seal's body, following the grain of the collagen within the muscles and running at 45 degrees to the seal's body. Seal deaths have been occurring over larger territories, including the north and east coasts of the UK, which suggests that the Greenland may be moving beyond its traditional habitat range. However, recent research into the UK "corkscrew" seal deaths by the Sea Mammal Research Unit  concluded that the UK seal deaths were unlikely to have been caused by predation from the Greenland shark, rather being caused by blunt mechanical trauma "consistent with the seals being drawn through a ducted propeller" that are found on many ships.
- Kyne, P. M., Sherrill-Mix, S. A. & Burgess, G. H. (2006). "Somniosus microcephalus". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. http://www.iucnredlist.org/apps/redlist/details/60213. Retrieved 05 February 2012.
- O'Donnell, Jacinth. Jurassic Shark documentary (2000); broadcast on Discovery Channel, August 5, 2006
- ADW: Somniosus microcephalus: Information. Animaldiversity.ummz.umich.edu. Retrieved on 2010-09-06.
- Florida Museum of Natural History Ichthyology Department: Greenland Shark. Flmnh.ufl.edu. Retrieved on 2010-09-06.
- Wood, Gerald (1983). The Guinness Book of Animal Facts and Feats. ISBN 978-0-85112-235-9.
- Caloyianis, Nick (September 1998). "Greenland Sharks". National Geographic, p. 67.
- Zoe Lucas. "Shark Predation on Sable Island Seals (July 2008)". Greenhorsesociety.com. http://www.greenhorsesociety.com/sharks/shark_predation.htm. Retrieved 2011-05-23.
- ""Clash of the fiercest predators as shark eats polar bear"". The Independent. http://www.independent.co.uk/environment/nature/clash-of-the-fiercest-predators-as-shark-eats-polar-bear-891512.html. Retrieved 3 January 2012.
- ""Greenland Shark" by discovery.com". Dsc.discovery.com. http://dsc.discovery.com/sharks/greenland-shark.html. Retrieved 2011-05-23.
- ""Video: Greenland shark at over 9100 feet deep (2770m) off Brazil" by thedorsalfin.com". http://www.thedorsalfin.com/shark-news-stories/greenland-shark-vide-over-9100-feet-deep-2770m-off-coast-of-brazil. Retrieved 2012-02-21.
- "Polar Seas: Greenland Shark". Elasmo-research.org. http://www.elasmo-research.org/education/ecology/polar-greenland.htm. Retrieved 2011-05-23.
- "Retrieved 20 March 2008". Sportfishingmag.com. 2004-12-21. http://www.sportfishingmag.com/species/fish-facts/shark-eating-35284.html. Retrieved 2011-05-23.
- Skipper Uses Knife To Kill 600-Kilo Shark[dead link]
- O’Reilly, Lindsay. "The Greenland Shark", Canadian Geographic, March/April 2004. . Retrieved 1 July 2007.
- "Greenland Shark and Elasmobranch Education and Research Group". Geerg.ca. http://www.geerg.ca/gshark1.htm. Retrieved 2011-05-23.
- "Greenland Shark". Marinebiodiversity.ca. http://www.marinebiodiversity.ca/shark/english/greenland.htm. Retrieved 2011-05-23.
- "GEERG: The Greenland Shark" http://www.geerg.ca/gshark1.htm . Retrieved 5 July 2008.
- Channel 5 documentary Retrieved 29 September 2010
- "Sea Mammal Research Unit" "Report on recent seal mortalities in UK waters caused by extensive lacerations", October 2010. Retrieved 5 January 2011
- "Somniosus microcephalus". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=160611. Retrieved 23 January 2006.
- Froese, Rainer, and Daniel Pauly, eds. (2006). "Somniosus microcephalus" in FishBase. May 2006 version.
- "Greenland Shark" on "As It Happens" May 6, 2008; CBC Radio 1(WMV file)