They can be as big as great white sharks, but that's about as far as the comparison goes. Their maximum speed is a lethargic 1.7 miles per hour, many are almost blind, and they are happy to eat rotting carcasses. They may be common throughout the ocean, but you've probably never heard of them. Meet the Greenland shark.

Looking like nothing so much as a chunk of weather-beaten rock, Greenland sharks (Somniosus microcephalus) can grow up to 7.3 metres (24 feet) long, making them one of the largest of all fish, and the biggest in the Arctic. But they prefer to live in deep, cold water, so humans rarely see them.

Studies in the Arctic have revealed a few snippets of information about Greenland sharks, and more data is now starting to come in from elsewhere. It turns out that Greenland sharks are bizarre, and may be crucially important for the ocean ecosystem.

Greenland sharks only come close to the surface in places where the shallow water is frigid enough for them –  primarily in the Arctic. They are most easily seen around Greenland and Iceland. As a result, they were long thought of as purely polar animals, as were the closely-related Pacific sleeper shark and southern sleeper shark.

But they have been reported on the coasts of Canada, Portugal, France, Scotland and Scandinavia. Some researchers think they live in many other areas too but just haven't been spotted in them yet.

"They may be everywhere that's cold enough and deep enough," says Aaron MacNeil of the Australian Institute of Marine Science, in Townsville, Queensland.

The obvious way to see a Greenland shark in the wild is to dive into the deep sea. For instance, in 2001 a remotely operated vehicle in the Gulf of Mexico captured footage of either a Greenland shark or a sleeper shark in over 2,600 metres (8,530ft) of water.

They may be everywhere that's cold enough and deep enough

Two years later, a pilot and a scientist from the Harbor Branch Oceanographic Institution in Fort Pierce, Florida, became the first people to come face to face with a Greenland shark in the deep sea. The shark, which was five metres (16ft) long, bumped into their submersible vessel 1,000 metres (3,280ft) down in the Gulf of Maine.

But hardly anyone dives that deep. So these rare encounters can't tell us how widespread and important the Greenland sharks are.

However, if the history of fishing is any guide, Greenland sharks are common as muck. The sharks were fished from the early 20th century until the 1960s; mainly for their liver oil, which was used as lamp fuel and industrial lubricant. In some years, over 30,000 were taken. That suggests a very healthy population.

In line with that, a recent expedition used 120 hooks on a longline, and caught 59 sharks. "I think they're fairly common," says Aaron Fisk of the University of Windsor in Ontario. "When we want to catch them we don't have any trouble."

So what are all these Greenland sharks eating? To find out, scientists have to get their hands dirty - by cutting open the sharks' stomachs and pulling out the remains of their meals.

It seems the sharks aren't too concerned about the freshness of their meals

So far this kind of work suggests the menu of the Greenland shark is highly varied. As well as fish, they eat just about anything that might fall off the ice, including reindeer and polar bears.

Given a chance it seems they will even try to eat moose. Last November, a man in Newfoundland found a Greenland shark gagging on a piece of moose hide, which had probably been thrown into the water by a hunter. He and another man decided to save the shark from choking on the hunk of moose. "A couple yanks and it just came right out," he told the Canadian Broadcasting Corporation.

It seems the sharks aren't too concerned about the freshness of their meals. Researchers have found small crustaceans called amphipods in their stomachs. These critters swarm over dead bodies in deep waters, so finding them hints that the sharks sometimes eat carrion.

That would make sense, because it's hard to understand how a Greenland shark could ever catch living prey.

For one thing they are absurdly slow, moving more sluggishly than any other shark. One satellite tagging study found that they usually meander about at around 0.8mph (1.1kph), accelerating to 1.7mph (2.7kph) when going all out. Others say they can reach 2.2mph (3.5kph). Regardless, many of the things they might want to eat can swim faster.

If that wasn't enough, many Greenland sharks appear to be almost blind. The culprit is Ommatokoita elongata, a crustacean with the nasty habit of permanently attaching itself to the front of the sharks’ eyes, damaging their corneas (see the photo above). In some populations, 90% of Greenland sharks carry these parasites. The shark that rammed the Harbor Branch submersible had them dangling from its eyes.

So how do Greenland sharks catch anything? It has been suggested that the parasitic crustaceans might be bioluminescent, and that the light they give off attracts fish for the shark. But that's "poppycock", says George Benz at Middle Tennessee State University in Murfreesboro. He says the scientific literature on the sharks is "contaminated" with unsubstantiated claims like this.

Benz thinks the sharks are more likely to be ambush predators. For example, Arctic seals sometimes sleep in the water to avoid polar bears, potentially allowing Greenlands to sneak up on them.

The seals also have to poke their heads through ice holes to breathe, giving the sharks an opportunity to catch them unawares. "They can still see light and dark, and a hole in the ice is like a big flashlight that says where the food comes in," says MacNeil.

No one has directly observed Greenland sharks catching seals in this way, but there is some circumstantial evidence. Large numbers of dead seals with "corkscrew" wounds have been recovered at Sable Island off the coast of Nova Scotia. Some were bitten in half, while others had the skin and blubber stripped from the lower halves of their bodies. "We're thinking those are Greenland shark bites," says MacNeil. "When they bite, the seal spins to get away, stripping the blubber away literally as it's trying to get out."

A hole in the ice is like a big flashlight that says where the food comes in

Others are unconvinced, arguing the seals were chopped up by propeller blades.

Whether or not the Greenland shark is the "Corkscrew Killer", Fisk has evidence that the species eats seals aplenty. He knew that seals in Svalbard have a short average lifespan despite seemingly ideal conditions: humans don't hunt them, they have few known predators, they aren't being accidentally caught by fishermen, and the area is not polluted. Wondering what was killing the seals, his team carried out stomach analyses on 45 Greenland sharks, and found that about a third contained the remains of seals. That was enough to explain the shorter lifespans.

Perhaps surprisingly, Greenland sharks may also tackle much bigger prey: whales.

Fisk's stomach analyses showed that they eat the discarded leftovers from Norwegian whale hunts. And it's not just dead whales they'll go after. Fisk has photographs of a group of beluga whales that came to a grisly end after becoming trapped by shifting ice off Baffin Island, northern Canada. "The polar bears went to town," he says, and so did Greenland sharks. "There were definitely Greenland bites on those beluga."

All this suggests Greenland sharks are playing a big role in the Arctic food web. If they are as common as everyone now suspects, they would have a big impact on other animals, says Fisk.

That may also be true in their deeper habitats, even if they get most of their food from sinking corpses. Benz says the sharks could be helping provide food for a wide range of other animals by breaking up these larger chunks of flesh. "A lot of organisms are going to benefit," he says.

If Greenland sharks are so important to the waters they live in, it would be good to know what is going to happen to them. "I think we need to think a little more about Greenland sharks," says Fisk.

In theory there are two things that could cause a problem: overfishing, and climate change.

Hákarl is either an acquired taste, or a contender for the most disgusting food on the planet

However, fishing seems unlikely to pose a major threat to the sharks. For one thing their meat is toxic, because it is rife with unsavory organic contaminants. So Greenland sharks are not regarded as a good dining option. In 1968, a group of sled dogs was fed Greenland shark flesh. Reportedly they were left walking stiffly, hyper-salivating and vomiting - not to mention having muscular convulsions, respiratory distress, and explosive diarrhoea. Some died.

A small number of Greenlands do get caught, to supply demand for an Icelandic delicacy called hákarl, or fermented shark. The meat is detoxified through a multi-week rotting process. Hákarl, MacNeil says, is an "acquired taste". Others have described it as a contender for the most disgusting food on the planet. It probably won't catch on enough to threaten the species.

Fishermen might catch the sharks by accident, though. From the late 1980s, the Inuit returned to fishing for Greenland halibut as a means of preserving their culture. Greenland sharks try to snatch free meals from the fishing hooks, and can get wrapped up in the lines.

That leaves climate change. Perhaps its most dramatic effect is the rapid retreat of the Arctic sea ice, particularly in the summer. What will that mean for the sharks?

As the summer ice levels decrease, the window for fishing grows larger. So while the halibut fishing has been limited so far, that could soon change. Large commercial fishing operators are well aware of this opportunity.

If we have ice-free summers, the food web could change dramatically

But the effects of the retreating ice go far beyond a few fishing boats. The entire Arctic ecosystem revolves around the sea ice. For the Greenland sharks, ice acts as a food delivery device. It's what keeps seals over open water, and as it melts it delivers dead animals as potential meals. That food source could be drastically cut as the ice shrinks ever further.

But other animals, particularly fish from further south, are migrating into the Arctic. Might the Greenland sharks start eating them? As so little is known about the sharks, it's difficult to say what will happen. All we can say for sure is that the Greenland sharks will be living in a very different Arctic in a few decades' time.

"If we have ice-free summers, as predicted in the near term," says MacNeil, "the food web could change dramatically."